"This classroom is very big."
I would use せまい or ひろい in this sentence. These are better for describing a space that you are in.
While inside a space:
This room is very narrow/cramped. このへやはとてもせまいです。
This room is very big/spacious. この部屋はとても広いです。
大きい and 小さい are better used for something that you are looking at from the outside.
While you are not inside a space:
That house is very big. そのいえはとても大きいです。
This apple is very small. このりんごはとても小さいです。
In a direct translation 大きい and 小さい are technically correct, but in this instance it doesn't sound natural.