Air compressors come in many sizes and varieties. The most important factor in choosing an air compressor is to determine the maximum CFM (cubic foot/minute) the tools that you are using require. CFM requirements are tool specific - for example, a paint sprayer or framing nailer will require a much higher CFM than a brad gun or a trim nailer. An air compressor with a larger tank will typically have a higher CFM because basically there is more air available for the tool to use. More specifically, the CFM output is related to the pump size. Features to look for when purchasing an air compressor include: oil-free vs. oil lubricated, noise, CFM output, cooling capacity, air intake filters.