What size breaker and wire do I need for my garage wall heater?
For a 26A nameplate load, Code will require #8 wire and a 40A circuit breaker. There really isn't any legal way around it.

Not what you asked, but by code you also must have 8 ft. from the bottom of the heater to the garage floor, because you have to avoid igniting any potential gas leaks from your car (remember, technically a "garage" is for a car, although few of use use them for that). If your garage has an 8 ft. ceiling, that isn't going to fit.