I just put a 7000 BTU window unit in the work shop room on my garage. I’m adding a wall and that room will be about 16x16 and the walls and ceiling will have 6” foam insulation. The trouble with garages is they normally don’t have any insulation and heat loss will kill you. The bigger window units that run on 110V are around 10,000 BTU. That’s about the limit on a 15 amp circuit. They will cool a pretty large area we have one that does our whole first floor, but again we don’t live in Texas or Fla. I would suggest a window unit even if you have to cut a hole in the wall for it. Where I live I went with the smaller shop and insulation because of heating for winter and the bonus is air con in summer.
The problem I see with sizing heat or AC for areas that only get used sometimes is the thermal mass problem. I used to have a wood stove in my old place in the garage and it worked great except it took 8 hours to get it warmed up in the winter. I would go out on a Saturday morning and fire it up and about the time I was done working for the day it was getting nice. The same is true for AC unless you want to run it all the time in a big area and that will cost a lot. With my smaller insulated workshop I have a place to work that’s cool or to go to cool off if working on something that requires the larger part of the building. Out there I will have fans only.