It is not possible to get to max size and also get the best ending. I wanted to include something for those who chose to forego the best ending to still find a fun thing.
Viewing post in Enter the Expansion Mansion comments
Unfortunate.
Still, a very complete experience. No complaints on the writing or how things go.
Only suggestion I'd have from here is the best ending including a new code to be put into the keypad that does let you get both maximum size and the best ending. But it'd require already being as big as possible for that bonus to work.