If your just going to run an impact now and then, a 15 gallon type should do fine. If you going to use steady running air tools like a die grinder, sander, or something along that line then look for 30 gallons or more. I dont know if your in the market for something new at all, but if so check all the reviews you can. There are a LOT of crappy compressors out there. If you buy a lightly used one make sure ask the original owner if it was broken in properly. That is one of the most common mistakes that new owners make, is not reading the manual. Almost all compressors need to be ran for 10 to 30 minutes (depending on manufacturer)with the relief valve open for proper break in and piston ring seating. Many people dont and you will get a compressor with a really short life span.
Unless Scout says dont do it. Then, really, just do it.
I know its blistering fast but you cant outrun ugly.