I was going to say 10'x10' would likely be about right but it looks like you beat me to it. If you are not planning on using it all that often & don't mind working in close quarters 8x10 might work but it would be pretty tight. Anything less seems like a waste of time & space because it wouldn't be big enough for much. If you are planning on an 8x10 you want to be sure there is nothing else in the room but what you are painting & you. Maybe what ever you are painting on a table with wheels but nothing around edges for sure!

I agree unless you are just doing it for very limited use. The more room the better if you expect to be using it all the time or a good bit but devoting 1/2 of a one car garage to a "spray booth" you intend to use once or twice a year seems like a huge waste of space. As first person to reply said 10x10 is about ideal but I think you could get away with an 8x10 if you put items you are painting on a rolling table and can move that table a foot or two during the spraying process.

One other thought would be to make "paint booth" bigger then have rolling shelving units you could roll out when you wanted to use the spray booth. (I use the wire rack style places like Sam's Club sell for my pinball parts). If I were to build a spray booth I only used a couple times a month I would store my parts (on rolling racks) in the spray booth when I wasn't using it for painting. That would keep my parts bins somewhere clean all the time and then before using the paint booth I would just roll the racks out into the shop and clean up the booth a little before spraying.