Shipping Rules - Packaging Size Improvement

Currently when creating Shipping Rules, if you want to assign a Packaging Size to that rule you can either add All Sizes or one specific size (e.g. Small Parcel). What I am hoping could be developed, is the option to select multiple packaging sizes, but without selecting all. e.g. I could choose Letter, Large Letter and Small Parcel, but leave the remainder unselected. This is especially useful for any customers who ship with Royal Mail internationally, where lightweight but large items can cause issues with shipping rules and shipping mapping.