In Birkenstock a "Narrow" width is more like a women's medium. A "Regular" width is like a women's wide width. They use Euro Sizing. For example a 39 Reg would be like a women's 8-8.5 wide. To figure out your size, add 31 to your shoe size and that will most likely be the size you wear. (31+8=39)
Birkenstock shoes run a little different in size than the sandals. I would size up. I normally wear a 40 in the sandals but I wear a 41 in the shoes.
NOTE: All leathers are different. Colors may vary slightly.