One of my bestsellers is the gym backpack or string backpack for school kids.
I have 2 sizes : Small 35cm x 40cm and Large 38cm x 43cm.
In the Large backpack a A4 folder fits well.
Another bestseller is the Toddler Backpack, sewing pattern from Made By Rae.
For a more modern model there is the Happy Backpack, sewing pattern from Ateljee Caro.