This is easiest done in Training Mode, as in real combat it's rather pointless and easy to make yourself SD.
Fight on the Eldin Bridge level with Jigglypuff. Wait for the bridge to break and get a Smash Ball ready. When you see the portal open up above the bridge area, immediately jump in the middle of where the bridge starts forming and use Jiggly's Final Smash. She'll start to get bigger as normal, but when the bridge solidifies, she'll get interrupted and lay stunned on top of the bridge at the same size she was at when she got hit.
Though she's bigger, her jumps cover the same small distance and she dies very easily if she strays too far to one side. If you have a Yoshi playing with you, you can make her ever larger at this point: Have him turn Jiggly into an egg and she'll grow a little bigger when she breaks out. Keep encasing her in eggs and she'll become bigger than the screen itself!
Submitted by Artemis251.