iRule to block Excessive cookie size

Well, you could technically use the (partially obsolete) string bytelength command:

when HTTP_RESPONSE {
    foreach x [HTTP::cookie names] {
        log local0. "cookie($x) = [string length $x[HTTP::cookie value $x]]"
        log local0. "cookie($x) = [string bytelength $x[HTTP::cookie value $x]]"
    }
}

But I venture that the ASCII length is going to be pretty close to the byte length.