Yes.
First off, let's note that there's nothing in the Druid description that specifically precludes the WS1 → WS2 transformation you're contemplating.
Second, consider this line of "Wild Shape":
You retain the benefit of any features from your class... and can use them if the new form is physically capable of doing so.
Wild shaping is a druid feature, so you retain it if the form is capable of doing so. Is it? For me there are 2(.5) reasons to suppose it is:
- Wild shaping does not require any spell-like components: gestures, utterances, &c. Some of those might have been impossible for your dire wolf, but they're not needed.
- We know a druid in animal form can transform: into a druid! The druid's personality, mental scores, and proficiencies all carried over. Absent a prohibition, it seems strange to think that this one part of a druid's core doesn't carry over.
- (Plus, if memory serves, Robyn--the druid queen of Douglas Niles Moonshae trilogies which informed much of the early D&D canon on druids--once changed from one form to another. The time she ran into Faerun's first peryton, IIRC. Can't put my hands on that book right now, though.)
Lastly, what would it harm to say "yes" to your player? The action-economy seems to be the only place where this could be exploitative. If we were to rule "no," then a druid has to use both their bonus action and their action to transform WS1 → druid → WS2. If we rule "yes" then a druid has to use their action to transform WS1 → WS2, leaving them "ahead" by a bonus action. In my opinion, that's a pretty small risk to run: the average druid doesn't have a lot they can do with an isolated bonus action.
It May Depend on the Creature (and the DM)
The rules are unclear on how high a "creature's space" is, and a DM may have to make a ruling. Unfortunately, "a creature's space" is defined only in a 2-Dimensional sense: on PHB, p. 191, it is defined entirely in terms of squares (not cubes).
Tiny: 2.5 by 2.5 ft
Small 5 by 5 ft
Medium 5 by 5 ft (etc.)
A DM will have to decide how tall a creature's "space" is when it comes up. This may be based not only on a creature's height (which might vary from about 4 feet for dwarves to about 8 feet for hobgoblins), but also by the space they control. As you stated:
A creature's space is the area in feet that it effectively
controls in combat, not an expression of its physical dimensions. A typical Medium creature isn't 5 feet wide, for example, but it does control a space that wide. (Ibid, bold added)
So precisely how tall a "creature's space" is may be different than its height. Different DMs may decide this differently (declaring a 5 foot cube to be every medium creature's "space" or determining space based on height, or however they wish). In some games, the top 5 foot cube of the door might be considered outside of an enemy creature's space (although definitely within their reach, and thus subject to opportunity attacks).
In your campaign, it looks like the DM considers the top part of the door to be within the hostile creature's space. Thus, unless they are using the optional "Tumble" rules (DMG, p. 272, which allows you to make a contested Acrobatics [Dex] check as an action or bonus action to move through an enemy's space), you cannot pass through it.
Best Answer
The spell Enlarge/Reduce can be used as analogy.
The content on Wild Shape in the PHB and Xanathar's Guide to Everything has no information about the possibility of insufficient space.
The spell Enlarge/Reduce does, however. The Enlarge part of the spell states, inter alia:
Of course the part about objects is irrelevant and the increase in size depends on the monster wildshaped into. But an analogy can be made for larger size wildshapes since this is an size-increasing and magic effect like the spell.
Of course, a change in shape occurs as well, making it closer to Polymorph and True Polymorph but those spells do not have a limited space clause.
Launching away creatures would definitely be a house rule since that kind of effect would need to be stated in the rules for Wildshape. As GM you can of course decide to make Wildshape work that way.