In reply to Rich_Garnett: Rich - I am 6'4" and bought the Osprey Mutant 38 (Large) for my most recent summer Alps trip this year. In fact the Large pack is 40L and expanded (it has a floating lid) significantly bigger - if anything almost too much space for summer Alpinism when staying in huts - but the back length was spot on and it is comfortable.
Only thing it's missing IMO are the pockets which appear on the waist-belt of other packs (eg. the Osprey Kode 30 which I ski with) and instead it has loops for racking gear. My preference would be for the former since I don't feel like I need the loops when I have my harness, but it's a matter of personal preference I guess.