I was also debating the install of a garage in-wall heater, and had seen the 2000W/240 amp version of that unit (for $84) I have a smaller, 20'X15' detached garage, and I figure that the smaller unit should be fine for me.
I think that the general rule is 10W per sqft, but it does depend on how well insulated the garage is, and what the outside climate is like (mild winters here).
-Dean.