The pottery barn kids backs are adorable and I think will fine for a kindergartener but was wondering if by 1st or 2nd grade they will seem to childish.

At that age, tastes and styles change yearly. Get a cheap backpack with your kid's current fasvorite character. Even a cheap one should last the school year. Repeat next fall with the new favorite.

what is an large vs average sized backpack? are we talking lands end small vs medium or medium vs small?

Arlington county K supply list says "large backpack" guessing it's bigger than what you needed. We sent DS to a full day Private PK this year and up-sized his backpack to a standard size at the holidays. esp. important think about needing to provide snack and maybe lunch each day and you'll want one bag to dump everything into to contain the spread of stuff that happens with little guys.