As far as general capacity, you should be able to comfortably fit (3) 8-9 lb butts on a single 18" rack or 6 racks of ribs in a rib rack.
I think you could squeeze six butts on a 22" grate and 8+ racks of ribs on each 22" grate. You can cook a serious amount of food on a 22" WSM. Just need to monitor temps/charcoal.
I've been getting ~12 hours out of a 20 lb bag of King blue. Maybe 10 or so with 70/30 mix of Wicked Good and King Blue (and smoke wood).
Since I have cooked a little more on my 22", I am starting to like it more and more. I still think the best cooker out there for a homeowner is by far the 18" WSM. It is pretty efficient on fuel, holds that 250°F temp almost automatically and can cook a decent amount of heat.
Of course, cooking in 50-70°F temps is a whole lot easier than worrying with the 30°F temps.