They are noisy when running, obviously this then becomes how often will it run, so then it comes down to size.

I would say you want nothing less then 20L, again how much you use will depend on how often it kicks in, I have a 5L one I use for blowing out dust on computers small things and so on and a 20L one for when I've got a "big job on", generally though it's the 20L one I reach for purely because of how long a full tank lasts and so to avoid the noise of either kicking in.

.Me