In reply to paulharmer:
Depends on the actual needs.
So what you actually want from the sack.
Something like REI Flash 18 (
http://www.rei.com/product/827110/rei-flash-18-pack ) would be stellar for some pure rock routes.
Something bigger would be naturally better for when you need to lug more stuff.
I've been rather happy with Blueice Warthog 26 (
http://www.blueice.com/products/en/home/10-warthog-26l.html ) as my main alpine sack (ie. need to lug axes with me).