I understand your looking for the size air compressor to pneumatic tools. I found a link while searching Google under EHow. This question will give you some instructions on how to pick the correct size. I attached the link below, I hope this helps!!!!!!!
It is good that you want to get the correct size air compressor for your air tools. This will eliminate disappointment later when you have to wait for the air-compressor to catch up because it is not big enough.
It will depend if you are going to use these tools occasionally or on a daily basis.
You will need to look at the tools you are going to use to see the volume of SCFM it uses and at what PSI. (SCFM) Standard Cubic Feet per Minute.
The compressor's are rated in two SCFM and PSI ratings. One at 40 PSI that will deliver X SCFM and one at 90 PSI at X SCFM . Most of your spray equipment is rated at 40 PSI and your Impact wrenches and sanders and grinders ect. are rated at 90 PSI.
So I would look at the tool you have that has the highest SCFM at 90 PSI to select the size compressor you need. This should be listed on the side of the tool on the model tag.
An example would be 1/2 inch impact wrench that has an average CFM rating of 4.7 would need a compressor that would deliver a constant 4.7 cfm at 90 psi.
The rule of thumb tells us in actual use you would need a compressor with a larger output of 4.7 cfm to keep up with the tool. As in the case of a body sanders that eat a lot of air it would have to be a lot larger.
I think if you got a compressor that puts out 5.0 CFM at 90 PSI it would handle most tools adequately.
I hope this will help in your purchase. I have included a link to a size chart that may help you.