Get the specs on all the tools you think you'll run at one time. If you'll be working with 1000 watt hologens for illuminating paint defects while doing swirl removals or working at night etc. don't forget to add them in. Generally you'll want to total up the total amp draw of all the tools that you'll have plugged in at any given time, including radios etc.
Take your total amp draw and multiply it by the voltage to get the wattage that you'll need to be covered for and get a generator that puts out about 1000 watts more than what you will be drawing. That way you'll have a little growing room and won't be constantly taxing your generator for all it's worth which would possibly burn it up over time.
so let's say you're going to run a 10 amp polisher, a 10 amp interior dryer, and sometimes have a helper running a 8 amp shop vac. so you have 28 amps total amp draw X 120 volts that equals 3360 watts. If you're doing this with a 1000 watt halogen lamp set up, add that to the total watts so 3360 + 1000 = 4360 watts. To run this load all the time and not overwork your generator you'd want to get at a minimum a 5500 watt generator.
I picked this one up for $300 with about 20 hours or less on it.