This will be based on the CFM (cubic feet per minute) requirements of the tools you’ll be running with your compressor. Larger equipment, such as sprayers and grinders, will require more air than smaller ones, like staple and nail guns. Check the tool itself or the manual to find out its CFM and PSI rating. If you’ll only be powering one tool with the air compressor, check the CFM/PSI requirement for the largest one and use that as your benchmark. If you’ll be using multiple tools at the same time, add them all up and then make sure you purchase an air compressor that produces a higher level of CFM.