Wow. I'm kind of overwhelmed by the response here. You guys have convinced me this is a bigger problem than I had really considered.
This Sunday I confronted the Bosom Bully and said very calmly that her public (and loud!) criticisms of my clothing were not appropriate. I explained that, although she was welcome to her opinions, expressing these opinions in such a public way would really turn off people who hadn't been in church as long as she and I had. She could have as many opinions on that as she wanted---I said this because she began telling me why she was right about "modesty"--but she also had to realize that not everyone grew up the way she did and we can't expect them to act as if they did. I also told her that, should she attempt such public criticisms in the future, I would sweetly but equally loudly and publicly remark that "isn't it lucky for us that, while people look at outward appearances, God looks at our hearts, and that our church knows that."
A lot of really good advice was given here. Thank you all. I should add that I'm not really comfortable addressing this with the pastor. Not because I can't say "bosom" in front of a man of the cloth. He's just very shy, and I -know- he'd hate to have to sit down with this steamroller of a woman and tell her what's what. It's his job, I know. But I suspect handling it myself is not only kinder to him but ultimately more effective.