Hi Art - Here is some good info about boiler sizing. Keystoker KA-6
If it were me, I would ask Keystoker if they could put a KB-8 stoker an a K6 boiler...they used to do this for steam boilers to give them some extra punch without the larger footprint. The Keystoker design is very rugged and simple, but if you get some coal that is prone to clinker it can cause problems when the stoker is running hard.
Do not base your boiler size on what someone else is heating with it. No two houses heat the same, and you are not exactly in the tropics...
You want to size the boiler to allow for less than ideal conditions, such as mediocre coal, or a boiler that is overdue for a mid-season cleaning. Don't worry about the boiler overheating, these stoker boilers will idle for days on end just like a big diesel engine...but without the smoke.
How warm do you keep the garage?