My boys are just starting to cycle daily to school and their old school rucksacks (cheapies!) are looking very uncomfortable. DS1 is 11 and 5ft 6 so adult sizes would be ok. He needs to take his school books and sometimes sporting kit in the sack. I went to shop this morning thinking it would be good to see sizes but the only ones they had were hydration sacks with very limited space to put any belongings. Does anyone have any recommendations regarding type and size? It's hard to tell how big they are on the web hence my wasted trip to the shop.