Small house furnace works great
that's what I have in my garage,but my garage is literally bigger then my house. I just have it plumbed into the gas line for the house. I can it to about 70 in about 15-20 mins. Or you could always get a heater that hangs from the ceilings like most professional shops have. I have seen smaller versions of what they have. I'm on my phone now, but when I get home I will post a link.