Air tank size

[1966ponyguy]

This may have been covered in past posts, I did look, but didn't find what I was after. I've order 4 2600's for our 68 swb. Have done 75 crossmember swap for front disc and swaped the rear to keep the 5 lugs all around. All bags will be mounted in stock coil locations. How much tank do I need. I have moved a S10 fuel tank between the frame rails, and have my crossmember cut and a pipe welded in to run the exhaust through it. Running out of room fast. I've seen everything from 2 1 gallon tanks to a 3 gallon tank used. Don't really want to mount tank in bed.