[Duratys]

Well first you need to decide what tools your going to use with it. Every tool has a certain amount of air that it requires to operate properly. If your compressor dosent "put out" equal to, or better yet more than, the tool your using you will have to wait for the compressor to build up pressure. For me....waiting for the compressor is like poking dull sticks under my fingernails...

So lets say your going to use an orbital sander that requires 17cfm @90 psi, a 1/2" impact that requires 19cfm @ 90psi, and a 3/8" ratchet that will need 15 cfm @90 psi.
You will require a compressor capable of a minimum of 19cfm @90 psi. A better bet would be to find something in the 19-20cfm range @100-110 psi in case you need it for a tool you curently thought you didnt need. Large compressors arent cheap so buying one that is "more than you need" is cheaper in the long run. Be aware of the duty cycle of the compressor as well if your going to base your decision on being just large enough for your largest tool. An impact or air ratchet is intermitent in use...while an air sander in alot more continous. If your compressor is too closely matched it will run all the time your using it. If thats the case make certain the duty cycle is 100%