Whether I'm buying for 2 or 10 I always get around an 8lb butt. Leftovers are great and keep for days.
Now, to get it done in the time needed, do it hot n fast, which is the only way I do butts these days. At 350F chamber temp you can do an 8 lb butt in about 4-5 hours, wrapping it at an IT of 170-180F. At 275F you'll going to add another 2-3 hours so you're cutting it close. Or you can smoke it for 3-4 hours at 275F, wrap it up and finish it in the oven at 350F. Put it in for an hour then check the IT. Final IT should be 200-205F. If not there just check it every half hour or so. Would definitely be done in time.
A smaller butt, say 5 lbs, would be done in just under 3 hours at 350F on the smoker, around 5 hours at 275 F, wrapping at the same temp.