It should be assur D’rabbanan for a shadchan, boy’s mother, or the boy himself R”L, to inquire about a girl’s dress size, or any other size for that matter. If she is overweight in an unpleasing way (to him), then he does not have to go out with her again, though he could be cutting himself off from a fantastic shidduch, if he would only give it a chance. Dress sizes go up and down (so do his pants sizes, btw, especially after a few really good home-cooked meals from a kallah trying to impress her new chosson).
We should be teaching our boys AND girls to be less concerned with externals,even as they have the right to feel a measure of attraction to their date. But do we have to make it EASY for them to be preoccupied with this naarishkeit, by making it a prerequisite, as it were, to being set up?