You should be more specific here, but this involves answers to some other question:
The surface is just the same surface limitations, The underworld is pretty much the same size as the overworld.
Probably about triple? Nearly everything is returning, with more varitaions, and different enemies in each biome. Overworld, Caves, Underworld, and Sky Islands all have their own sets of enemies for the most part. For example, caves in the early game have had various types of Horroblins, Like Likes, and the glowy frogs.
when you say each biome has new enemies, does this hold true for the stuff on the surface? Does the snow area have snow monsters, death mountain fire monsters, etc? Hugely exciting if true