For a shoe to fit perfectly, it must have the proper length and width - which is why the Birkenstock foot bed is produced in two widths.
When making your purchase, please take note of the correct width symbol: regular width for normal or sturdy feet, narrow width for slim feet. Your feet should sit embedded in the footbed, meaning that you must not be standing on either the inner or outer side of the footbed rim. In addition, you should have several millimeters of space in the heel and toe areas to allow your feet to move freely and naturally with each step you take.