For blowing out water lines, just about any size (1gal+) will do. As mentioned above, the smaller the tank, the longer it will run between faucets to build backup to (output) 30-40 psi. IMO, it is more important to make sure you have a hose/adapter configuration that will allow you to connect the compressor to your city water input without having to hold it in place.
I took a ~6" piece of old garden hose (male end), used a hose clamp and an air tool 3/8" male quick disconnect at the other end. At the end of my compressor's hose I have the female quick disconnect fitting. I connect the hose end to my camper, inner lock the quick disconnects and turn on the compressor. That seems to work great.
TV: 2008 Ford Expy
Camper: 2012 Rockwood Roo 233S
12V: 2 x Group 27 plus Honda Ex 700c Genset