I finally learned the truth. There is NO standard in shoe sizes. I wear a size 7 in heels, and in ballet flats, I HAVE TO SIZE UP. And after wearing birkenstocks for years, my feet is wide compared to the widths of normal shoes. Basically shoe sizes depend upon each individual shoe, the style, the heel height, and the materials it's made from.

I found that I need to wear size 8.5 in order to fit well in these shoes.  Try on the shoes at a store first, then decide where to buy.