Family Reunion

I am experiencing a lot of anxiety about my family reunion.  I am bipolar as well as having an anxiety disorder.  I know that my anxiety is from not being sure how to respond to certain overly-critical family members.  But I don't want them to have the upper hand.  I need some advice, please?  How do I respond to overly-critical people? I mean for things like "My you've sure gotten fat, haven't you?" kinds of questions or comments.  I don't want to ignore it, but it I don't know how to respond without being really bitchy towards them, making me look unreasonable and petty.  I envision being calm and rational, and having the perfect thing to say back to end those kinds of comments while keeping my self respect intact.  But what sorts of things can I say?

Thank you in advance for your help and advice,

A.