whatever backpack you buy, if the brand doesn't give a guide about fitting it on its website, be suspicious.
for the best fitting backpacks, go for brands that makes them by sizes, and not for those who make them for variable sizes - and be sure to get the right size for you.
this type of brands will probably be american, like Osprey or Gregory.
Osprey Packs
Gregory Packs
will be your next problem...