Got it.
Here is a prototype:
blankrow.knwf (80.0 KB)
This will work on the assumption that the sizes will not repeat… i.e. in your example mini is only the first 4 rows and mini does not appear again further down in the table.
The workflow iterates over the different groups of sizes and then inside the loop “flips” the table 90 degrees moving row indexes to columns and column names to row indexes. Then a column “Blank” is added before it is flipped again to return to original column names and with one additional row (blank column from before) at the bottom.