It all depends on how much air you need or what you’re going to be running. If it’s just a small 4 or 5 bay self-serve you can get by with a 20 gallon 100% duty cycle air compressor. If you have an automatic then you’re going to need something like a 60 gallon or an 80 gallon if you’re going to be using a lot of compressed air. The slower an air compressor runs the longer they will last. A lot of manufactures use the same pump but increase the pump speed so the air compressor will put out more CFM’s. Don’t EVER! think about buying an oil less air compressor, they are total junk, won’t last long in a car wash. In 1994 the night before we were to go on vacation the air compressor at the car wash died. We scrambled around and found a 20 gallon Sanborn at Builder’s Square, got there right before they closed. We installed the Sanborn thinking it wouldn’t last long. It’s still in service today, replaced the pressure switch once and rebuilt the motor once, I’m waiting for it to die, but it just keeps on chugging along, it runs real slow.