I'm probably overlooking the obvious, but where would I enter dimensions of inventory? Does it require any module other than the Inventory Management module?
Are you looking for "dimensional inventory" attributes? i.e. This sheet of Item 123 in 100cm by 200cm but another sheet of the same item in inventory is 400cm by 600cm.
Or are you just looking for some fields to stuff the standard height, width, thickness, etc of an item?