I think most of it comes down to insulation. My 1979 built fully finished garage is about 22ftx20ft. I bought this Hair portable 12k btu unit for it about 5yrs ago but it struggles to keep it below 78 if the outside temps are over 90F
http://www.amazon.com/Haier-Commercial-Conditioner-CPF12XHL-UV-BR-REFURBISHED/dp/B0096E712G
Turns out that while the one wall and the ceiling that I finished with R19 insulation the other wall doesn't have any insulation behind it, this also happens to be the wall that gets the most sun in the afternoon. Also the roof doesn't have enough vents so the attic cooks to above 120F.
BTW I went with the portable unit because at the time the DIY mini splits weren't common and I didn't want to have to modify framing of the load bearing wall with jack studs and headers etc to make room for a through-the-wall ac unit.