Try it all, you're sure to get something cool! I've made a lot of tin can cameras and have done a bunch with students and I've found that in terms of getting evenness of exposure having the paper curve with the best. That makes sense, keeping the paper as close to equidistant to the pinhole negates light fall off. As far as size of paper in the can I'd say as big as can fit, the angle of view will be very large. This may be just me but I found that I like more than one pinhole in the can, usually three works pretty well, images overlap, you get some pretty interesting stuff.