-Map Pieces can be no bigger than 896x896 units. This doesn't mean it has to be a perfect square. As long as it fits within those units.
-Keep textures simple. You aren't keeping these pieces anyway so don't go to town on aesthetics. Whoever gets your piece can do some of that!
-Pieces will be quality controlled and possibly rejected for the following reasons
The map piece is just a big empty square.
Incredibly convoluted with a ton of 3D floors.
It's straight up unplayable
-Please refrain from adding scripts, weapons, and items to your piece. Adding a full fledged gimmick to a map piece is a little crazy, and weapons and items can easily be subject to change in the final map. People are also able to add their own gimmicks and whatnot to the map pieces when creating the final map.
That's all for the map piece side of things!