In reply to Doms1:
I'm 6'7" too. Any pack less than 19'-20" back is way too short, which rules out a lot of packs, especially under 40l.
I have a size 3 Pod Black Ice which is long enough, but is a good 55l or so.
http://www.podsacs.com/i/q/CCPBIBP/pod-black-ice-backpack
Cilogear packs also come in sizes and L is long enough for me.
http://www.cilogear.com/45lws.html
Osprey Variant is a bit heavy and overdone for a 50l pack but their size 3 usually fits me well. The old Mutant 38 size L was good but now in the new model they only make two sizes and the larger one is marginally smaller than the old L. I have an Osprety Variant 28 size L which is excellent, but they're no longer made.
In the UK, Aiguille can probably do minor mods
http://www.aiguillealpine.co.uk to make a pack longer.
I had a custom job done by Randy Rackliff at Cold Cold World. He can make several modifications, within limits, and I had an Ozone made with a 22" back, which takes it to about 40l.
http://www.coldcoldworldpacks.com/ozone.htm
I use it as my crag sack, as it's simple and durable to take hardware. Every time I put it on I realise just how much better a pack feels when it is long enough and fits properly. If the length and basic design/dimensions are right then you need almost none of the straps and other adjustments sold on most packs. But if you want a 21" back in a pack less than 70 litres, this rules out a lot of packs.
Post edited at 02:50
