Bought a Float Throttle 22 this year and love it. I thought coming from a non avi pack that it'd be pretty heavy, but really didn't notice it on at all. I added a tank bag to my sled, so most of what I carry went in there.
I carry shovel, probe, saw, first aid, and cpr mask in my pack along with a bottle of water and some food incase I'm to get separated from the sled over night and still have plenty of room. I could probably get away with an 18, but I never had the chance to see any in person or try any on before I bought, so I went bigger just incase. I'd recommend taking what you plan to carry in your pack to your local dealer, stuffing it all in, and wearing it to see how it feels before purchasing.