Enter these codes at the versus screen:

Unlimited Turbo: X, X, X, X, Y, Y, Y, Y, B, B, B, B, DOWN
Softball Mode: X, X, Y, Y, Y, Y, B, B, DOWN.
Ball Is Rubber: X, X, Y, Y, Y, Y, B, B, UP.
Max Out Speed: B, B, B, LEFT.
More Time After Each Play: X, Y, Y, B, B, B, LEFT.
Big Heads: X, X, RIGHT.
Tiny Heads: X, X, LEFT.
Max Out Batting: X, X, X, LEFT.
Max Out Power: Y, Y, Y, LEFT.