With our first two children we go the smaller backpacks......the we not happy about having to fold everything they made to fit into it and we ended up with either an unhappy child or having to carry all their projects home. They also didn't allow a pocket folder to fit in there without having to fold the corners down and that made them wear out faster. We didn't have to carry all the extra clothes, ect. back and forth. Just left them there. But, if you have to keep that in there, then a small back pack is not going to work at all for you. Also, they had nicer backpacks that lasted for several years and when they got to kindergarten, we were told to get a bigger one to fit folders in better anyway. It ended up being a waste of money.
Our 3rd child (youngest in her class), we used a regular sized, but light weight backpack. MUCH better choice. She looked like she had a turtle shell on all the time (lol), but she never complained about it. Everything always fit into it. All three of my kids were (and still are) small for their age. I don't think it is as hard as people think it is for kids to carry regular sized back packs.