You don't mention where you live.If you live in Dallas, you have less concern than if you live in Fargo. Insulation is an issue and hopefully you have your garage insulated.
You don't mention why you want your garage heated. If it is to keep ol' Nellie warm for easy starts, that is one thing. If you plan to live in this space, that is another.
Capacity of you existing unit is important. If it is already running at capacity, you will have problems keeping the remainder of your house heated and cooled.
There are other issues, but you can resolve most of these by having a "reputable" heating and air conditioning contractor review your situation. He will be able to tell you if need an additional heating unit of some kind. For no more space than you are talking about, a thru-wall heat pump base board units may be the ticket.
Good luck, BEG
PS. If you do live in Dallas, let me know if you are the JTaylor who is my cuzin in law.