You’ll want to come up with a theme for your level. What would make it interesting enough that your friends will want to play in it? Even a normal platform with a long row of spikes on either end is somewhat memorable, as it allows players to push their opponents into treacherous territory.
Another popular example of this is the “Pinball” stage. Place two inclined surfaces toward the bottom part of the stage, ramping down to the middle of the screen. Have each ramp lead to a giant spring that acts as the pinball flipper, propelling the player to the top of the screen. The rest of the stage is comprised of springs, spikes, conveyors and other fun surfaces.
Since you cant change your level size once you start building, you may have to start over if you find that your stage is too big or small. In this case, just consider your design as a first draft, and fine-tune it the second time around.