Ours are size small for our bike touring gear. For mountainering with a larger bag and tent we go up to medium and large. I think MTB's Idea is a good one for getting the right fit. IMO, it is better to have the sack a little on the small size than have a larger one that is only 3/4 full. With the larger sacks they end shaped like a volley ball instead of tubular. FWIW we have Mountain Hardware, Granite Gear, and REI compression sacks; and they are all comparable. REI also has an outstanding return policy.

I don't leave a wet sleeping bag or tent in a compression sack any longer than necessaary. However, I've never had a problem using them.

There was a whole tread about using compression sacks tents on a ski mountaineering forum. Like most threads on any forum the opinions were about equally split on it does no harm vs. sure destruction.