MM, I think you are giving her remarks far more attention than they deserves. The first thing you should do is stop counter-arguing with her. Dismiss her remarks for the silliness they are, when it fits, and tell her to stop commenting on your body. Tell her this in the moment, in front of the same people she makes her comments in front of. She will only be embarrassed if she chooses to embarrass herself by making her remarks in front of other people.
Sometime during the second week, I needed to get by her. I said, "Excuse me," and she stood her ground. Jessica said, "Nope, you can't get past me." It was obviously a game, so I didn't mind, but I did eventually get past her, smiling, and said, "Ha, I got past you!" Her reply? "Yeah, because you're skinny."
MM: "You say that like it's a bad thing."
Michael was showing her things by using me, and once she did it on me, she said, "You're too little. I can't bring my elbows in like that, you're just too little."
MM: "Better work on that, then. You won't get to choose the size of the offenders you have to deal with."
She searches me out then finds a way to work how small she thinks I am into the conversation. It's getting exhausting.
"Co-worker, you make a lot of comments about my body. Please stop."
After that, you can upgrade to "Co-worker, you keep making comments about my body. It is rude, unnecessary, and inappropriate to the workplace. Don't do it again."
I recommend that you avoid the phrase, "It makes me uncomfortable," as that is exactly what she intends to do, and you will just be feeding her behavior.