Of your garage is about 20 by 22 feet assuming about a 9 foot ceiling. This is about 440 square feet floor area. I would make an educated guess that a 30,000 btu/hour furnace would do a good job for what you want. Without knowing the R values of your ceiling, walls, etc the length of time to warm is hard to calculate. BUT I would GUESS about 1 hour to go from 25 to 65 and stabilize there.
Forced air would circulate better than baseboard electric, and stabilize the entire area quicker. Plus, you will need about 7000 watts of electric resistance heating to equal the output of a 30,000 btu/hour furnace at 80% efficiency. That will take a 40 amp circuit at 220 volts. The initial outlay for the electric will be less, but it will cost more to operate.