Two dimensional players:
Enter PRPPAPLYR as a cheat code from the main menu. Alternatively, press Up, Right, A, Right, A, Down(2), Up(2), Down during game play
Fat and thin players:
Enter ABBTNCSTLO as a cheat code from the main menu. Alternatively, press A, Left(2), Right, Up, B, Right during game play.
AL and NL All-Star players:
Enter BEST as a cheat code from the main menu.
Big heads, feet, and bat:
Enter GOTHELIUM as a cheat code from the main menu. Alternatively, press Left, A, Right, Down, B, A, Left(2) during game play.
Baseball trails:
Enter GRTBLSFDST as a cheat code from the main menu. Alternatively, press B, Left, Up, Right, Down, B, A, Left(2) during game play.
Giant baseball:
Enter BBNSTRDS as a cheat code from the main menu. Alternatively, press Up, Right, A, Left(2), B, Up, Left(2) during game play.
Little Homerun Derby Guy:
Make sure you have a roster slot open on a team. It should say "empty" instead of a player's name. Go to HOME RUN DERBY and pick that guy. You won't even see him walking in the grass because he's really small. It takes a while for him to reach the mound, so be patient. Watch the pitcher; he'll actually have to throw to the ground so this guy can hit the ball.
Cheap Fielding Trick:
Here's a nifty little underhanded way to cheat in All-Star: Wait until you're at bat, hit the ball anywhere, and then quickly pause the game and switch your controller to the other team. That way you can make the outfielder miss the ball, the SS miss an easy grounder, and so on.