Minimum size.
It seems like a lot of folks here cook smaller pieces of meat than what I consider optimal. I bet most wouldn't cook an entire rib. So do a piece more in line with what you think might be a more common size, like a 3-5 bone, down to the point where it just wouldn't work.
FWIW I don't think I'd do less than about 8#, but that's likely still too big for most folks.