read my post above

/oxide.grant user <user name> backpacks.use.large ​

But if you havent granted that players group permission (default is the "default group") do that first

/oxide.grant group default (or group name) backpacks.use​

Its all pretty well explained in the plugin overview (documents) page. Hope you get it going its a good plugin

And apart from your own backpack key binding the bindings for the other players who cannot open their backpacks must be done by them,