MB_LEN_MAX - maximum multibyte length of a character across all locales
The MB_LEN_MAX macro is the maximum number of bytes needed to represent a single wide character, in any of the supported locales.
A constant integer greater than zero.
POSIX.1-2001, POSIX.1-2008, C99.
The entities MB_LEN_MAX and sizeof(wchar_t) are totally unrelated. In glibc, MB_LEN_MAX is typically 16 (6 in glibc versions earlier than 2.2), while sizeof(wchar_t) is 4.