I'm having the same issue.
You can refine by size only if the seller has denoted a Size Type (eg.petite, plus, normal) on their listing first, which over 90% of the listings in women's tops haven't done.
The assistance I received from ebay chat today, was to ask the seller of the item I wanted to view to refine their listing to include a size type!
No shopping for me on ebay today then, as I don't have time to trawl through all 640,000 listings for women's tops that have unspecified size types.
