Most Birkenstock styles are available in 'regular' or 'narrow' width.
Narrow width is suitable for narrow to slender feet.
Regular width is suitable for normal to wide feet.
If you already own a pair of Birkenstocks or are trying them on in store - check out the little foot symbol on the footbed. If the foot is coloured in, it's a narrow (N). If it's just an outline of a foot, it's a regular (R).