How many BTU should a furnace have to heat a 1000 sq ft non insulated cinder block building?

So I have a garage that is about 1000 sq ft (I know it's large for a garage). So anyways I was wondering how many BTU a furnace would have to be to heat this garage. It has cinder block walls that are not insulated and a roof that is also not insulated(but easily able to insulate). It's also not very tall (about 10ft to roof). I'm not looking to heat this thing all the time(that would waste a lot of energy) but I do want to be able to turn a furnace on and be able to comfortably heat the garage for a few hours(or how ever long I happen to be out there fixing something). Also it gets pretty cold here like -25C sometimes. Thanks for any help /suggestions.