Big Duke, Little Duke
There is a way to play big or small in Duke Nukem. This code will work on co-op or deathmatch. To do this code in co-op you need Invincibility and items codes. Turn on friendly fire, then use the weapons code, shoot your buddy with the shrinker, or expander.
Small Duke: Shoot, put on invincibility then turn it off again. The player will not grow back (the drawback: you can't have any weapons).

Big Duke: Get the expander, (with God mode off) and shoot the other Duke until he starts to expand. Just before he explodes put on God mode, stop shooting, then turn off god mode. You now have a giant duke that still can be killed. (Note, you may have to try it a few times to get it right.)

Midget Duke Execute the Big Duke trick, then turn off God Mode and shrink the big Duke Nukem. He'll now be smaller than normal but larger than "Little Duke."

Come Fire Walk With Me
You can safely walk on lava for a infinite amount of time by pressing the jump button enough that your feet aren't on the ground for more than a second. Keep it up and you can stay in lava without protection for as long as you want.