I bought the 15-inch for a Canon 70-300mm zoom and the 11-inch for a Canon 20mm f/2.8 prime. Both fit perfectly. To size them, know this. The 11-inch is 11" wide on all four sides, so it is 15.5" diagonally (high school math and the Pythagorean theorem for the win). Look up your lens, take the maximum width and double it, take the length and double it, then add the two up. It should be a little less than the diagonal measurement so you have some overlap for the velcro to grab on.
I bought two of these for a trip, so I could take a couple lenses but not the whole gear bag. They worked very well, but you'll still have to pack them properly. I wouldn't just toss them in anywhere. On the plane, I had them wrapped up inside a jacket in my backpack. Walking around, they were snug inside a shoulder bag with a few other things. I wouldn't just roll them around loose in a big backpack, as the padding is not super thick. I see some folks have used this for wrapping a SLR body with lens attached, but I don't see how. There are better products for that, check out Zing neoprene wraps - they are custom sized for a better fit.