I have a 30' x 80' shop where I ran 1/2" galvanized pipe throughout. Things to consider are as follows: 1. PSI is PSI and is constant all through a system, it's only the volume of available air that changes when two or more people are using the pressurized air system, then you may want larger pipe. 2. Black pipe was designed for natural gas usage and can rust when hot moist air from your compressor is run through it. I found this out the hard way when I went cheap and used some left over black pipe in my a/c system. The rust particles would clog my air filtration units and needed often to be cleaned. There are many opinions regarding air line systems, but this is what works for me.