Trjtdtk

As a Loxodon Druid I plan on "hulking out" into an Elephant using Wild Shape.

The question came up: If I'm in a small corridor and I Wild Shape into an Elephant, can I choose the size of the Elephant to be small enough to fit in the corridor? Is there anything in the rules saying you assume a certain size based on the beast? Not necessary going from huge to tiny, but do I get to choose where on the "huge" scale I end up?

To clarify: I'm not interested in changing from "huge" to "large". If huge is 16-32 height/length* do I get to choose how big within that in that range I end up? Could I shift into a 16ft long elephant with one use and then a 30ft elephant with the next use?

Kind of, with a little wiggle room

I'm assuming your druid is Circle of the Moon since without Circle of the Moon, you are limited to CR 1 wild shapes maximum, per the Beast Shapes table:

2nd | CR 1/4 max | No swim or fly speed | Wolf

4th | CR 1/2 max | No fly speed | Crocodile

8th | CR 1 max | Fly or Swim speed ok | Giant Eagle

Circle Forms for Circle of the Moon says:

The rites of your circle grant you the ability to transform into more
dangerous animal forms. Starting at 2nd level, you can use your Wild
Shape to transform into a beast with a challenge rating as high as 1.

Starting at 6th level, you can transform into a beast with a challenge
rating as high as your druid level divided by three, rounded down.

A circle of the moon druid can transform into a beast with a CR of 4 (elephant) at level 12 and above.

Wild Shape says you use the stat block of the creature you transform into:

Your game statistics are replaced by the statistics of the beast, but
you retain your alignment, personality, and intelligence, Wisdom, and
Charisma scores.

An elephant is a Huge Beast according to its stat block. So if you turn into an elephant it is always a Huge one. The wiggle room is that you could probably make the elephant vary in size within the range of Huge for flavor, with DM approval, but it will always be huge. This means that you are subject to the restrictions of a Huge creature.

No, the druid assumes the beast's size and other physical statistics.

When a druid uses Wild Shape, most of their stats are replaced by those of the beast.

While you are transformed, the following rules apply:

• Your game statistics are replaced by the statistics of the beast...

The beast's size is part of their statistics. So if you Wild Shape into the form of a huge-sized elephant, then you become huge-sized.

However, note you can squeeze in spaces that are one size smaller.

A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that’s only 5 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it’s in the smaller space.

For example, a druid Wild Shaped as a huge-sized (15 ft wide) elephant could squeeze through a large-sized (10 ft wide) corridor, moving at half speed and incurring some penalties. But they could not squeeze through a medium or small sized hallway.

If you are looking for a RAW answer to the extent of allowing no content of any kind not officially handed down by WotC then no. As several others have said, you can squeeze an elephant at half speed through a ten foot wide space assuming headroom, but it is a huge creature whether you want it to be or not. But rules are not the same as content and (outside of organized AL play) even RAW assumes the creation of custom content.

It never specifies in the rules of wildshape that the beasts must always be chosen from entries in the monster manual with their exact, unmodified stat-blocks. It never even references the existence of such a book. It does say on page 6 of the monster manual that a DM should "feel free to tweak an existing creature to make it into something more useful for you" and pages 273-83 of the DMG are all about how to do so.

So long as care is being taken to create a balanced creature, if your DM wants to stat out "young elephant" for you and give it a CR that you can wildshape into then that is not even really in the realm of a homebrew rule. It is how the core materials are intended to be used, as it is content creation for a campaign rather than a rule change.

The polite thing to do would be to offer to create a draft version of the creature and save your DM the work. Don't go crazy with it, but establishing that elephants start as babies and grow into adults in your campaign setting is not exactly going crazy.

This turns out to be unclear in the rules, and there are no official clarifications. However, there are at least three interpretations I've seen used, two of which I think our very reasonable and one I understand but do not think is actually well justified.

Interpretation One: generic beast of a type in the Monster Manual

I'm putting this one first, even though it's the one I like the least. The ruling here is that you choose a listing from the Monster Manual (or other accepted official source of beast stat blocks) and become a generic, average example of that beast. There's no room for customization, and you can't ask for an animal that exists in the world that does not have a stat block. By this ruling, you can't choose the size of the elephant form you assume.

I don't like this because the Monster Manual explicitly says:

Other Animals: A book of this size can’t contain statistics for every animal inhabiting your D&D campaign world. However, you can use the stat block of one animal to represent another easily enough. For example, you can use the panther statistics to represent a jaguar, the giant goat statistics to represent a buffalo, and the hawk statistics to represent a falcon.

... which at least implies that "reskinning" should be an option, and of course there's the whole "Creating a Monster" section of the DMG — this isn't supposed to be a game limited to those pre-made stat blocks. However, I have seen this interpretation many times and particularly in Adventurer's League play, where DMs don't want to have to deal with possible outside-the-box thinking. As written, the feature is very open-ended, and this is an easy simplification.

Interpretation Two: exact creature you've seen

This is a pretty straightforward literal reading. The rule says:

Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before.

... and by this ruling, you must have seen the exact form you want to observe. If you want to be an elephant, it needs to be the form of that particular elephant you saw out on the savanna on your journey there three years ago. By this ruling, you may be able to have a variety of elephant forms at your disposal, depending on your character's history.

The funny thing is, despite how straightforward this is, I have only seen the 5E druid played this way once, and that was because the player wanted to — they'd tell the story of the time they met this giant crocodile as they'd shift into that form. And they made a big deal of keeping a journal of beasts encountered in the campaign. But the DM for this game wasn't enforcing this at all — they very certainly would have allowed (and maybe sometimes preferred, when it got to be a bit much) a more loose interpretation.

Interpretation Three: seen one, seen 'em all

This is the most free interpretation, and in my experience the one most long running non-public-play games go with. Here, if you've seen an example of a beast, you now have unlocked the general category of "elephant", and can take an elephant form colored by whatever detail you like (much like a spellcaster assuming a different humanoid form with alter self). By this ruling, you can pick any size you like within the "huge" category.

Usually, this is also different from Interpretation One in allowing you to choose a totally different type of elephant with a custom stat block, as long it's reasonable to have encountered that alternate type and the DM agrees. This could be the difference between the real world's Asian and African elephants, or it could be "Juvenile Elephant" (size large, or even medium).

This idea isn't without backing in the rules; cribbing from my earlier answer to a related question, Xanathar's Guide has a section named "Learning Beast Shapes", which doesn't resolve this issue, but does casually mention:

The tables include all the individual beasts that are eligible for Wild Shape (up to a challenge rating of 1) or the Circle Forms feature of the Circle of the Moon (up to a challenge rating of 6).

... emphasis mine. The table does not, actually, list "Bessie, the mule from Farmer Grumwold's place up near Meadowville" as an "individual beast". It lists "mule".

This supports the idea that intention behind the rules is that "a beast" means "a type of beast", not actually a singular, individual beast.

In conclusion

There is no conclusion. :) You'll need to decide this at your table.

In a game that I ran, I had a druid who wanted to only specialize in dinosaur forms, but there turn out to not be as many of these as is representative of real-world dinosaur diversity, and some of them are weaker than traditional standby druid bear and wolf forms for no real reason. So I let that player come up with a whole suite of different dinosaurs at each CR, and made sure they seemed reasonable with the CR rules in the DMG (plus comparisons with other beasts at the same CR). This turned out perfectly well: their theme was fulfilled and they were happy.

It sounds like you have a theme idea going on with your loxodon. This is awesome. Were this my game, I would absolutely encourage you to have a selection of elephant and elephantine forms available to fit all of your Wild Shape needs.