I have mine in a 20x30 with a 12' ceiling. It is mounted at a 45 deg angle at the narrow end (20') of the garage facing the garage door at the opposite end. It seems to heat well but this is the first year that I have insulation and keep it running all of the time. We have not had extreme cold weather to speak of but I do not anticipate any problems with it keeping up. I do have a fan and have not noticed any cold spots, but the fan circulates the air. Even though the IR's are supposed to heat objects and not the air, there is still heat that they generate that heats air. I had a thermostat mounted close to the ceiling and the temp difference from that to 5' (thermostat) off the floor was about 20 deg's after it had ran for a while. The fan takes care of that.
I have a friend that has a body shop and has Reverberay units throughout and in the office, the office area has a 9' ceiling. The receptionist says it makes her head hot when it is running while at her desk. They are very quiet, I was hoping for that but the sunstar is very noisy. If I had to do it again, would have spent a bit more on the reverberay or went with a forced air unit. You do have to account for the clearances.
Mine is run off a line voltage thermostat, there was an option for a 24 volt transformer if you wanted to run a 24 volt thermostat. I think all dual stage units run 24volt thermostats, mine is a single stage and the line voltage control seems to work fine. You are limited to pretty basic thermostats though when using line voltage.