This question keeps coming up in our groups over and over without a satisfactory conclusion (and frankly quite a few hurt feelings on the part of upset polymorphed players, and the rules lawyers constantly trying to get them to waste a 4th-level spell slot).
What size doors can a Giant Ape (MM, p. 323) squeeze its body through? It is listed as a Huge beast.
I think the question really might come down not to "what is the exact diameter of the ape's head / pelvis / collarbone" but actually one of: "What are the dimensions of the average dungeon door?" Though I suppose some concrete answers to any of these questions would be better than what we've currently got.