What is your goal with this heater? Your comfort? If so, it's important to keep it close to you. There are two ways that heat moves: radiation and convection. The radiation range on that thing is only a few feet, so you'll need to be close to it to get any comfort from it. And convection from a heat source that small isn't going to heat your garage quickly no matter where you put it.
For safety sake, don't mount it somewhere something might fall on it or touch it, so that probably means above head level. Maybe above your work bench pointed down?
Also, for safety sake, put it on a timer that runs no longer than 30 minutes and does not have a bypass or On setting. That way if you forget to turn it off, it will turn itself off and hopefully not burn down your house!