I took the M size with me on my last trip as hand luggage, which was a bit too big, and it really does fit. As a backpacker, I wouldn't take the L, as it gets heavy over time and is not very comfortable to carry if you have to walk for a long time.

I'm flying to Peru/Bolivia this Friday for 3 weeks and will be doing two treks there (1x 5D/4N and 1x 4D/3N). I bought sizes M (which also includes sleeping bag and trekking shoes) and S (which also comes with the backpack on the treks) as check-in luggage. The 95-litre bag can hold an incredible amount. If you're not travelling with it all the time and always have to lug it around, this version should definitely be enough.