Nelco, I don't. Just try to leave whatever size or shape you do postcard size. (Give or take a little here or there.) As far as different shaped, your only limited by your imagination. I decided to do different sizes and shapes on a regular rectangle postcard. But, you don't have to do that, You can actually shape your card if you want. I did think about that also. I just couldn't wrap my mind around the concept.
But, examples of shapes could be Oval, Round, Butterfly, Kite, Car really, the possibilities are endless. I look forward to seeing what you come up with.