I put this frame bag on a Trek Stache 9.7 size small. I got the small bag and it fits snugly. The exandable nature of the bag is good, and when expanded, it fills the main triangle completely. I can put in my frame pump, a thick pair of winter gloves, tire levers, multitool and a tube. That pretty much fills all the space. Attacment is easy, although the front and rear belts have a long floppy end that needs to be tucked in somewhere when the are tighthened up. I just wrap the loose end around the frame and tuck in the strap. The bag has performed well in 0 degree winter weather and so far the zippers have worked fine. It does not get in the way of peddling either. S
This is a really nice bag, but even the Medium-sized version was too big to fit my Rockhopper 29er. The price was very good - I saw the same bag in a retail store for 50% more.