It's a unary operator, kind of like ~ or !, which can apply to parenthesis (), until it meets a type, then the parenthesis are mandatory:

    int x;
    sizeof x;     // okay
    sizeof (x);   // okay
    sizeof int;   // not okay
    sizeof (int); // okay

    read_prefix():
        is peak() '!': ...
        is peak() '~": ...
        is alpha(peak())
            maybe alpha is sizeof?
                wait, it wasn't sizeof? it was an identifier? long rewind
        is peak() '+': ...

\ramble on

(actually, better yet, it should've been used instead of & for taking the address). Imagine:

    int x;
    int* p = @x;

Ive actually toyed with the idea of having @ as a pointer dereference. It seems to make sense: you want to know what is at the address stored in p.