You can use any size butt you'd like. I just use a 15 lb butt per gallon of brine... Its more as a reference... so If you have a 8 lb butt use about half a gallon of brine/cure. The crucial thing is to make sure that the meat is totally submerged.
And btw... I love butt hams.
They are sometimes called "Cottage Hams". However a true cottage ham is the butt trimmed to just the money muscle (neck muscle at the top of the butt). I just debone mine and tie the whole thing up nice and tight.