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.