If your feet are unaccustomed to the contour footbed, start gradually. Birkenstock footwear comes in different widths. Most styles come in Narrow and Regular. The Narrow fits Narrow to Medium width feet. The Regular fits Medium to Wider width feet.
Some styles only come in Medium width.
The Medium is in the middle of Narrow and Regular , it fits most people but if you have a very narrow or very wide foot it may not work for you.