When sales associate at Victoria Secret asks your gfs size is it rude to say that she and your gf look about the same size?

Rude? Probably not, but also no too helpful. Best to know before going in or you'll get something that your gf won't like all that much.

Women have vastly different shaped and sized breasts and what seems to look the same on one will likely be uncomfortable, the wrong cup size, etc., and you've got the wrong thing.

Not at all.