Minecraft wiki entry for History_of_world_generation, associated text: Map sizes (width × length × height)
Square Long Deep
Small 128 × 128 × 64 256 × 64 × 64 64 × 64 × 256
Normal 256 × 256 × 64 512 × 128 × 64 128 × 128 × 256
Huge 512 × 512 × 64 1028 × 256 × 64 256 × 256 × 256