In reply to nbrowne:
Also consider if you want to climb with the sac. I had a Black Ice size 2. It was about right for me at 5' 10", but the length of the sac back meant I couldn't look up with a climbing helmet on - they interfered with each other and ice climbing was more difficult.
Now have a size 1 and it's much better, for me. Smaller volume also suits my needs better!