I bought the large one and it fit my 102 liter (6200ci) backpack well. Although you cannot put the cover on backpack if something big, like sleeping pad etc, is fixed on it externally. It's not an issue since 6200ci backpack is big enough to cram all the stuff into it.

I haven't examined cover's durability and I haven't had the cover in rain. However it's apparent that the cover is waterproof. But I'm not sure what may happen in rain. A backpack will probably be dry from the front, top, bottom and sides - the parts which are fully covered by this raincover. But what about the back which is open?? won't it get wet?? I don't know yet. When I try it in rain conditions I'll update this review.