Drumroll...Size 6, but a deflated bubble...

Here's another way to look at it: She has no experience with this staying off. We've all dieted and failed. Maybe she can't just wrap her head around it, and she doesn't want to see you upset. Ask her, and tell her how she made you feel. Don't let the feelings build up. Just be nice about it and tell her. Treasure your mom. Yes, words can hurt, but be thankful you still have her to go shopping with. (Stepping off soapbox. Please forgive me. I miss my mom and our shopping trips.)

Buy the size you're most comfortable in. You look beautiful, by the way.