if you use 房间很小了 to describe the small size of a certain room. i think it's fine but it's not proper enough. this sentence 房间很小了 has a potential meaning that the room had been arranged by somebody and become small finally. as well that, i don't suggest you to use the sentence 房间多小了 either. actually, the meaning of 房间多小了 is that "what is the size of that room?" after a certain period, a decoration or something else. well, my friend, i believe you can figure this out all by yourself in the Chinese chat. i wish i could help.