19x28 might be a little too big for a rear rack on a folder though. I'm hoping to have the cart strapped down on the rear rack while I ride. So, preferably, the cart shouldn't be any bigger than the rear rack if it's possible.

How big are the rear racks anyway? Anyone know the aprox dimensions of the built in rear rack on a folder? I know it's going to vary greatly between the different models, but since I've never seen one in person I really have no idea how big they are.