[rdc12]

So in practice it works more like a C Macro then a C function?

Edit, except it doesn't treat arguments the same, so no.

[cygx]

It's neither:

sizeof belongs in its own category of unary compile-time(-ish) operators, whose only other member is _Alignof.

If you're so inclined, you might add _Generic and _Pragma to that mix as well, but they aren't really a close fit.