Best practice for addition of items

We would like to know best practices for addition of items in the system. We have item like Paper (A4 size, A5 size, A3 size). Do we need to add these 3 items like Paper A4 size, Paper A5 size, Paper A3 size individually or Paper will be single item added into the system where as size will be mentioned on requisition (Additional Description).

We have similar items like medicines (10mg, 20mg etc) or Sanitizer (100ml, 250ml).