there is a tool store in western Canada, KMS TOOLS, that has an online guide to compressors. maybe a good read.
https://www.kmstools.com/assets/cate...uyingGuide.pdf
you need to know what tools you may be running and then figure out the cfm (cubic feet per minute) that each one will gobble up while using it and the duty cycle that the compressor will need to keep up that rate. duty cycle is how often the compressor is made to run. some are continuous and some are less. something like an air nailer would need way less air than say a body sander because the nailer just uses a small amount at a time while the sander uses a lot all the time. then factor in the voltage you have available to run it with and the shop space you have available for it to sit in. once you have a compressor you will wonder why you didn't get one earlier. my compressor is a fairly large one and I have never regretted getting it. no more waiting for the compressor to build before I finish something.