We originally ordered the medium for our 4th grader. It was way too small to fit gym clothes, lunch, binder w/ folders, text books, library books etc. We now have the large and while it is big for his tall but thin frame, it has plenty of room for all he needs and he will be able to use it for many, many years. Customer Service for the exchange process was wonderful. They were completely interested in us being completely satisfied-something most companies don't seem to care about these days.
My daughter loves this bag. At first the material seems thin but it's high quality. My daughter loves the many compartments and the strap is very high quality and adjusts quickly and easily. The padding in the strap is great.
As usual, the quality of the LLBean bag is great, my only complaint is that the strap cannot be shortened like a real bike messenger bag so it is always flopping around my daughter's hip. I wish it could be cinched up so she could easily ride her bike to school without it falling forward.