To alter your character's appearance, follow the instructions below while selecting a fighter.

To change your costume:
Select your character with CIRCLE

For Meaty Arms:
Hold L1 and L2 while selecting your character.

To change your costume and have a big head:
Hold L2 and select your character with CIRCLE

To change your costume and be a kid:
Hold R2 and select your character with CIRCLE

To have the same costume with a big head:
Hold L2 and select your character with X

To have the same costume and be a kid:
Hold R2 and select your character with X