Serious bulge watching is a good way to give off size queen vibes. Also, checking out his crouch before you check him out in any other way is a size queen signal. You must make sure he sees the bulge watching. It should work to scare off many small guys and some average size guys.
Won't work on all though, as some guys just want to "get theirs" and don't care what women think about it. And some average size guys really believe they are hung.
Plus, from your bulge watching you may even be able to discern if he has some extra size going on. It won't be a guarantee he'll be big when erect, but it will increase the chances.