After watching a bunch of YouTube review videos, I've decided to go with the large one. One question though, the Navy color seems to be varied from site to site. Some sites lists it as uncle blue which is the same color with the pic on description. Others list it as NaVy which is a bit darker like the one you see at checkout. So I'm unsure which one we're getting. I prefer the darker color though.
http://www.fjallraven.us/products/rucksack-no-21-medium?variant=1284458880