Considering the two construction options < SMD and double-sided > separately, I prefer SMD every time over through hole. I dread drilling lots of holes; although, I am set up well to do it. 1206/1210 SMD are only a little smaller and actually don't save much space, but IMHO are easier than through hole. 805-size SMD is the smallest I go. They are not much harder than 1206, once you get used to them, and they save lots of space. As for the chip, an SOIC-sized chip saves lots of room. Without a socketed chip, though, you will probably want to enable ICSP for programming. As for eyesight, I am not a youngster. I wear glasses and a head magnifier.

As for the board, for those who use the photo method, double-sided and single-sided are not that much different. Just tape the transparencies together, slip the board in, and flip the unit after exposure to get the other side. However, for those using toner transfer, I can appreciate that a double-sided board would be more difficult. However, isn't the eventual plan to provide a commercial board? That would be so much cheaper and more convenient for everyone; everyone except for the poor guy who agrees to coordinate it.

Thus, if the plan is to use a commercial board, which will include a solder mask, I think SMD with 805 or 1206 is the way to go. I would keep the processor to an SOIC like size for ease in soldering.

John