Silly Backpack Questions

We are getting DS his first backpack. He will be going to "zoo school" once a week (basically preschool at the zoo) and will use it for that and all the other things we do on a daily basis (carrying anything he wants to take with us for playdates/errands/to Mimi's, library books to story time, etc...). I'm wanting something that fits him well, is not to big but will hold what he needs, and will hold up nicely. What size does your 3 or 4 year old have? Brand recs, etc... What do you feel is a reasonable price? 
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 I have showed him packpacks at Target and a few other places and while he is super excited about the idea of a backpack he is not in love with any of them. He loved the monkey ( from Skip Hop I think) but when I asked him if that was the one he wanted he said no. He has mentioned colorful planes or motorcycles several times but I've looked everywhere and no dice. Anyone seen one that has colorful planes or motorcycles on it? I found some SUPER cute ones on etsy that DS did seem to like but the dimensions are about 9' high and that's probably not big enough, right? I'll probably end up finding a few that look good and let him pick between the top 3 on my list since "color motorcycles" is not something I'm finding...   OR maybe I'll get a cute, simple, plain one and get him a patch....