I use a small 5 gallon husky that's easy to transport. It really depends what you using for. I just use it to blow out trim, wheels, and to blow out and clean my polish pads. If you got say a tornador than you need something much larger.
CFM...(cubic feet/per minute):
•Select a compressor with enough cfm to support the most demanding air tools you have.
-Every air tool has an operational cfm requirement
-Every air compressor has a maximum cfm output.
-The air compressor’s cfm output must be greater than the air tool(s) cfm requirement...to keep the air tool(s) operating efficiently.
PSI...(pounds/per square inch):
•Compressors (and air tools) have a psi rating that should also be taken into consideration.