I LOVE Ecco shoes (they have saved my back and my pain doctor actually recommended Ecco 3 years ago).  And I have always worn size 39 but when these sandals arrived, they were way too narrow so I had to send them back.  And I usually take a medium width.  Too bad because they're cute but I'll keep looking.  By the way, I can't comment on the arch support or how long I could wear them comfortably because I didn't walk in them.  I just tried them on and saw right away that they were not going to work because of how narrow they were.  Happy Shopping!