Pause the game by pressing P, then type in all caps the code that corresponds to the desired effect. (Thanks to ALMEHDA@aol.com)

Effect	Password
Big Feet	BIGFOOT
Big Hands	BAFFE
Big Head	BIGHEAD
Easy Mode*	FORTYTWO
Shrink	DWARF

*-Press Ctrl + D to refill your health. or D to lower your opponent's life.