If you were able to give each combination an item ID (outside of PBI desktop), and then use the sort column based on that specific ID, this would work.
For example,
| ID | Item - Classification | Sizing - Classification | Desired Order |
|---|---|---|---|
| BDK | Bed | King | 1 |
| BDQ | Bed | Queen | 2 |
| BSK | Box Spring | King | 3 |
| BSQ | Box Spring | Queen | 4 |
You would then create the order column based on ID and "Sort by Column" with this the order.
If you only sorted on the "sizing" classification, all Kings will be grouped together, all Queens will be grouped together, etc. (which I'm sure you've already seen).
Another way to accomplish this (depending how you want to do it) would be a custom column using DAX that would look something like this:
(For just itemorder)
OrderColumn = IF(table[Item] = "Bed", 1
IF(table[Item] = "Box Spring", 2
...................)
OR
(For item AND size order)
OrderColumn = IF(AND(table[Item] = "Bed", table[Sizing] = "King"), 1
IF(AND(table[Item] = "Bed", table[Sizing] = "Queen"), 2
...................)