Heater placement

Install a Carbon Monoxide detector alarm in the garage also for your protection and if a gas leak ever happens (hopefully when not being used) you will have a warning instead of whenever you might get around to going to the garage again.
My dad in KS had an older NG heater w/fan in a 25 X 30 cinder block 2 car detached garage and placed it in a front corner angled diagonal to the opposite corner and warmed it in no time with no "hot" spots....equal temp everywhere after 2 warming cycles. 8)

If I ever have the opportunity to have a nice detached garage, I will also go with a similar NG heater as shown in link a few posts ago! For an attached garage, I would run a vent from the house HVAC along with a moveable radiant heater for backup....IF I stay in FL.