Besides interior volume, the sizing can effect how the pack rests against your back and the positioning of the loadbearing straps. Go with what they recommend.
that's the thing, I don't see any sizing information online other than 16L vs. 20L...it's not like they sell a S/M and a M/L for each volume. I only noticed the 5'7'' comment on the tag for the 16L DLX. I never noticed on other Dakine packs so I guess I'll have to go back to a store and hope to find something in stock so I can check the sizing.
I can't imagine all 16L packs are for guys shorter than 5'7''. That's a pretty small dude to make a whole line of packs for.