We did this two places on the farm, the difference is we bought dual units, air conditioner and heater.
The shop unit is 240 and heats and cools about 650sf...the ceiling is about 13 feet.
It works great. I augment with fans in the summer. The unit is pictured below. I used to use propane heaters but didn't like the smell.
At the stable, we have a window unit that we framed in. It cools and heats about 300sf perfectly and runs on normal power.
I admittedly hate the humidity in Maryland so the cooling aspect is as important as the heat. Both units work very well and are only turned on when needed. I couldn't be happier.