I have a 50 litre compressor - a Draper, its fine for tyre inflating etc but not up to running a 1/2" impact gun. If I was buying a new one Id definitely go bigger.
(I have a nice litte 18V Ryobi impact wrench which is a nice bit of kit.)
I have found that I need a minimum 1/2" bore hose (depending on length) to run an impact tool.