I picked up a 10 gallon oiled compressor from Harbor Freight a couple years ago. It was supposedly rated for something like 5.8 cfm @ 90 psi and I picked up a Stanley impact gun (looks just like an Ingersoll Rand Classic) that was supposedly rated to use 5.3 cfm @ 90 psi. Figured that I'd be good, right? Nope. That thing will suck that tank dry faster than it'll fill. And up until I removed the stock regulator and let my impact drink straight from the tank, it didn't have enough stones to do anything. Now it'll do most stuff, but it still sucks the tank down in a hurry, it runs way better off of shop air (like a 60 gallon or larger tank running around 100 psi).
If you're looking at running an air chisel, sander, or painter, you need a much bigger tank than 10 gallons. And don't go cheap on the tools either. I made the mistake of buying some HF air tools.... waste of time and money (although the do have a nice deal on Goodyear rubber air hoses when they're on sale).
I'd say go with an absolute minimum of a 33 gallon, but realistically you'll want a 60 gallon (look for a used one on Craigslist, they pop up from time to time for reasonable prices, try to find an oiled one rather than the oilless design, oiled lasts longer). For doing more than small panels at a time or touch-up work, you'll want even more air on tap than 60 gallons though (I could cry every time I think of air compressors, I missed a deal on a nearly new IR 80 gallon - rated for something like 23.7 cfm @ 90 psi for like $700).
