1. Mark the size as 2 or 3 to indicate the number of pieces in the outfit.
2. Use the leggings size for outfits with leggings.
3. Mark the size to go along with how the albums are arranged in the multis they participate in the most often.
4. Mark the size for the focal piece in the outfit.
There is no wrong way to do it!
Finally, in the description of the outfit, consultants can put the full details of the size & style of each of the items in the outfit.