Go to the Create A Player screen and enter these as the first and last names of the player (when you want to go back to a normal game you will have to reset your PSX).

BIGHEAD BOBBY           big heads
COINHEAD COREY          2-d flat heads
LAMEBOY LENNY           backwards animations
GEORGE GIRAFFE          long necks
BIG BEN                 big characters
TINY TOM                tiny characters
MONKEY MICKEY           long monkey arms
SHRIMPY SEAN            short arms