Pixel a scene that is exactly 3 times wider than it is high - So, for example; 64 x 192 or 50 x 150.
Even though the canvas size is unrestricted, it is advised that you stick to relatively small sizes in order to avoid biting off more than you can chew...
Canvas Size - Unrestricted - but width must be 3 x the height.
Colours - Max 32.
Transparency - No.
Animation - No.