You can use bulk actions to set the vendor or size for multiple products at a time from either the Inventory > Inventory Items or Orders > Build Orders page.
Important Tip: You can select all applicable products by checking the first checkbox in the blue header row of the table. This select all checkbox obeys any filters you have set above the table. For example, if you filter by Type: Wine and click to select all checkboxes, only wines will be selected and updated. Since wines and spirits often come in uniform sizes, this is a quick way to bulk assign sizes for every product in your inventory very quickly.