I realized that players without backpacks.use.small but with backpacks.use CAN use a small backpack. I would like that players without the permission backpacks.use.smal (or medium/large) don't have access to the command /backpack but that they can loot backpacks on other players bodies. Is that possible? (I don't test this myself because I don't have anyone right now to do that :/ )

Anybody can loot dropped backpacks. The permission "backpacks.use" grants access to the default backpack size which is set in the config.

Oh, ok. Thanks!