Whenever I try to go into a portal that changes my size, the game crashes. I think it might be something to do with pehkui, which I do in fact have in the mods folder, but I don't know how to fix it.

Post the crash report

Seems that it's due to pehkui has an incompatible update

Does it crash with 0.90? https://github.com/qouteall/ImmersivePortalsMod/releases/tag/v0.90-1.16