If your garage is 28X36X10, guessing the ceiling height X about 7 gives you approximately 60,000 btu's needed to heat the space. If the "R" value of the insulation is good, that should do it. Personally, if you haven't poured the floor yet, use the floor heat. Wersbo, pex what ever you want to call it. A little pricey to install but real cheap to run. And if you're on the floor, warm as toast.
My two cents. And X 2 on the vent free. Combustion causes a lot of moisture.
Agree. A "word" or two about BTU
Electric is figured directly from wattage and is essentially 100% efficient other than a VERY small amount lost in the wiring.
But if you buy combustion, whether gas, oil, etc, pay attention to what the specs "are."
It used to be "usual" for the nameplate listing on gas / oil to specify INPUT BTU from which you have to figure AFUE (efficiency) to get "output" or usable BTU
On older gas, this was average about 70%, somewhat higher for oil
Only "a few years ago" there came about so called "80% class" which nowadays are about the lowest efficiency you can buy. These are all vented with conventional metal vent systems
And the newer so called "90 % plus" or "condensing" furnaces run up above 90% efficiency. These are good and bad. The good of course is reduced fuel. The bad is, some of these are / were unreliable and "ate" head exchangers. You can very quickly "eat" any fuel savings by spending money on service and parts.
I "hear" that "our government" will soon expect us to buy ALL 90%. These can be good and bad in a possible FREEZING environment like a garage. Since these condense the moisture out of the vent gases, (to recover more heat) this means there's a certain amount of WATER in the system. If you have a garage situation, and it's not always kept above freezing, this can be a problem
"Life's a crap shoot."
The point is you need to know what somewhat is quoting you, is a fuel unit OUTPUT BTU or INPUT. My neighbor fell victim to this through Lowes and very nearly ended up with a "way too big" furnace.