problem : sales order while creating unit of measure

My scanaio is like that, My material unit of measure is KG which may be 120gm, 250gms, 300gms, 500gms which are piece item like detergent cake etc, but according to sales policy, they sell the material in different bag size which contains many no of material.but bag weight is fixed like 17 kg, 21kg, 42 kg , 45 kg, which contains many no of detergent cakesso on. how to bring bag size while creating sales order

thanks

kumarn