The way I see it, you have 2 choices.
You can either heat with electricity or natural gas (or propane, if NG is unavailable). The first step is to size the load. In the Midwest, the rule of thumb is 40- 50 btu/ sq ft depending on the windows and insulation, etc., but a garage doesn't need to be 73* all winter long, so it's OK to downsize a bit (assuming you know there will be days the garage won't be 73*). OTOH, there's no point in spending money and not getting enough heat. It stinks to work in the cold.
The average one car garage is about 13' x 24', or 312 sq ft. If it isn't insulated, nothing you throw at it is going to seem like enough. By insulated I mean at least R11 in the walls, at least R25 in the ceiling, double pane windows, and insulated doors.
Assuming it's insulated, 30 btu/ sq ft is going to be adequate the vast majority of the time, and that attached one stall garage is going to take about 9,400 btu/hr to heat. If you have a 2 car garage, it'd be something in the neighborhood of 17,250 btu/hr, and a 3 car garage would need about 23,000 btu/hr. If the building is not attached, it's going to take more.
Electric heat is rated in watts or Kw. 10 btu/hr= 2.93 watts, so that single car garage is going to need 2754-ish watts. A readily available 2500 watt 8 ft electric baseboard heater (which puts out 8533 btu/hr) would probably do it, assuming you're OK with 60* on a sub-zero night. It would take 2 of the 8 ft 2500 watt heaters to do a well insulated 2 stall garage, and two 2500 watt 8 ft heaters and a 1500 watt 6 ft heater or three 2500 watt 8 ft heaters to do a 3 car garage. (All of this is very rough, and depends on your desires and climate).
The bottom line is that heating with electricity is the opposite of "buy once, cry once". It's dirt cheap to put in (wire and $90 heaters), but costs a fortune over the long haul. The difficulty with gas is that anything with a heat exchanger and flue is going to be at least 40,000 btu/hr, and cost at least $800. Then you have to run gas, electric, and a flue.
Enter the 99.9% efficient catalytic garage heaters. They need no power, and they run on gas. They come in small sizes, and lots of them run on propane, so all you need is a bottle.
My advice? DON'T DO IT.
Let's assume that your space is well ventilated (it's not, unless you are uninsulated, and then you are just peeing in the wind), and that you are one of the people who tell themselves, "that smell's not so bad". You'll think it's a cheap way to go. I was one of those guys. I had a 20,000 btu NG catalytic ventless heater in my 2-stall garage once. It was a huge mistake.
If you ever paint anything, that open flame with no flue is going to react with the thinner and overspray and put out some unbelievably noxious fumes. You'd be surprised how often you want to paint something, or clean parts with a distillate. If the garage is attached, the entire house will smell like a chemical dump for a couple of days. Worse, you'll likely be hacking up some pretty nasty stuff for a few days, assuming you aren't overcome by the fumes and don't die in the garage. It's just bad.
Please don't do it.