Who wants to learn yet another obscure Super Mario Brothers
Once you're done swimming around for forever in the minus world, hit reset and give this one a whirl.
Go to one of Bowser's levels. Get a mushroom so you're big (but not fiery), then go fight Bowser. You know that axe you have to hit to knock the bridge out from under Bowser? Don't hit it yet. Just stand next to it. Bowser should walk towards you. (Doesn't he look stupid in this game?) Wait until he is right next to you.
What you'll want to do is jump and land so that you hit both the axe and Bowser at the same time. If you do it right, Mario will flash as if he had been hit, but he won't shrink. He will continue to flash as he talks to Toad.
Now, you'll start the next level, and you'll still be big. Go into the level, and the next time you hit a block with a mushroom/fire flower, a mushroom will come out. Get the mushroom. Instead of making Mario big, this time it will make him small. (The effect is kinda neat, actually.) Now, continue going, without getting hit. The next time you hit a block with a power up, it will contain a fire flower. When you grab it, Mario will change to the fiery colors, but he will still be small. Congratulations, you're now small and fiery! Whenever you shoot fireballs, though, the animation will change for that frame to show the big Mario, then it will change back to the small Mario. I guess no bug is perfect.