For me the question would be how this material is stored in the warehouse
Is it just in pieces, or is it already packed in smaller and bigger cartons, or even already fully wrapped on a pallet?
If you store it in pieces and the cartons and the pallet is just a transport packaging and not considered as product cost, and even breaking cartons or pallets and repacking it on customer demand is not any issue ,then you can go with one material using alternative units.
But if the product is already stored in cartons and pallets and you want see how much of each size you have, then you should use extra material masters. And if it even matters for your sales department and the customers, production and costing whether you have 6 or 12 in a carton, then it even more speaks for extra material numbers.