I used to get 5-7kg shoulders from Trowbridge Bookers all the time, once 'freed' and rubbed up I would just tuck the two biggest sides underneath when putting on the grill and it makes a neat lump.
I have a UDS which is 57cm across and I can get two of the big shoulders (5-7kg) or three of the medium (3-5kg) ones on one shelf. Go for it, get the biggun
Bookers in Gloucester only do little shoulders, don't think I've had one over 3kg from them, which is a shame but my butcher is awesome, I just can't always get to them within working hours
Go on, get a big shoulder, get it on the smoker at midnight, then in the morning before you go to work pull it off the smoker, wrap it and get it in the oven at 120c, then when you get home, take it out, leave it for 2 hours and pull it.
MMMMMMMMMMMMMM