uid_t is 32-bits so the maximal length in decimal is 10 digits.  MAX_UID_T_LEN, which represents the maximum length of the decimal string representation of a uid_t, suitably defaults to 10 characters.

However, the rebuild_env() function in plugins/sudoers/env.c causes 10-digit uids to be truncated in order to nul-terminate the string.  One more character (for the terminating nul) needs to be allocated for idbuf in this function.