Buy Shipping - where is the size/weights table

Trying to use “Buy Shipping” in light of VTR problems. First went through slowly and filled out parcel size/weight for items which took long enough. Noticed that there was a link to a table to add item size weights and made a mental note to come back to it. Buying shipping for the second item I expected to be allowed to add the (different) item as a separate entry. Filling out the size/weights however created a “Custom package” rather than the item description as the identifier which will be confusing over time. I would like to be able to populate the table with the correct details but cannot find any reference or link to it. Does anyone know the link details.