I would not fret about the size of the butts you can find there. I used to use "boneless rib tip trimmings" that I found at a local grocery. They were pieces of pork that ranged from pork chop size to about twice the size of my fist. They made great Q. I'd stack the smaller pieces so they would not cook too much faster than the big pieces. My criteria is that there be a reasonable amount of fat in the meat. I pass on pork tenderloins for that reason. Tried one once and it was a bit dry.
An advantage of smaller butts is that there will be more surface for the rub and smoke absorption and they will cook in less time.