You really to gauge your hose size by the ID, which is inside diameter.
5/16 ID will flow just fine, and pretty much standard in body shops. 1/4 will work and if thats what you got I wouldnt worry about it because its only 20 ft long.
At 20ft and 1/4 ID you wont notice any PSI and CFM drop to affect a da.
But your compressor is under powered and the da is using more air then what it can produce.
