I love the Birkenstock Madrid style.  I was very disappointed in not being able to find medium width in all the amazing colors so I thought I would try the narrow size knowing that I could return them if they didn't work.  They fit perfect.  I am a medium in all my shoes but the narrow in the Madrid style fits me perfectly.  If you have never bought Birkenstock before the foot bed melds to your feet after wearing them.  They fit and hug your feet like a glove.  Try the narrow if you wear a medium just to see if it would work for you.  I love having so many more choices in colors now.  These are my favorite summer sandels.