Actually it's volume and it's completely artificial (e.g. plant spike has 1, minigun 6).
Beside this I think it's good idea.
But some item apper to not have any volume. For example ammo
I vote for implementing this, even if its a volume. It could give us at least some idea.
Maybe it could be reworked a bit, so that you can put more ore in a Hummer than in characters inventory, which is ridiculous in my opinion.