As an operator rather than a library function or syscall, 'sizeof' does not have a manual page. Its use is however not always completely obvious and especially novice programmers misunderstand its function. Being able to type 'man sizeof' to get an explanation is a lot more convenient than having to rely on Google->StackOverflow and hoping whatever answers one finds there is reasonable.
If the FreeBSD Project sees value in this and would like to add this manual page to the base system, I'd be happy to sign over the copyright.
Note: my mandoc-fu is weak, and fixes and improvements may well be necessary.