Hi,
Please check how the material to be valuated / tracked in SAP - whether you want to valuate large, medium and small boxes separately. If so, either you should have different material numbers to differentiate large, medium and small boxes or you can check the concept of split valuation, where you can go with single material and different valuation type.
If you dont want to valuate / track separately, you can go for different line items in the PO with same material number and description as large, medium or small, with corresponding price.
Regards,
AKPT