Go to any level as Peach. Now, get a mushroom or a poison mushroom. While you're small or big, whip out some items (down+B). The turnips will be super big or super small, and stay that way when you go back to normal size. Sometimes, Peach will whip out a Mr. Saturn or Bobomb instead of a turnip. It'll also be real big or small and stay that when when you throw them.
Submitted by: Jesse