I have a cousin-in-law who is getting married to my husbands best friend. My cousin and I have never gotten along, and I was surprised when she called me out of the blue for my measurements. I gave her my dress size, and the conversation was over. Apparently she later called my husband and told him that he needed to make a down-payment soon for my bridesmaid dress and shoes, he agreed to make a payment, and she ordered the dress. My problem with being her "bridesmaid" is she never once talked to me about the wedding, much less asked me to be a bridesmaid, and I'm pregnant right now (which she is very well aware of) and will be 5-6 months pregnant when she has her wedding. Is it rude of me to tell her that I do not want to be her bridesmaid?
The time to tell her that you weren't interested was when she asked for your measurements. But, you could tell her that you don't think that you would be able to fulfill your duties as her bridesmaid and apologize.
If she has ordered the dress, you would be obligated to pay the entire amount. After all, you let her to believe that you would participate.