Ornella, I often avoid small shops for this reason. Upscale boutiques are often quite expensive for my budget and are less likely to carry my size.
I've been working on saying no beautifully in recent months. I include a nice smile and a "thanks so much for help," and sashay out the door. I do feel like an actress playing a role. Once I actually, in my head, gave myself stage direction, "Smile. eye contact. exit quickly stage right." This goes hand in hand with my SING acronym!
On a related note, I said a warm "no" when approached to teach in our church's vacation Bible school today. Of course, I did offer my two children as my as stand-ins. I hope that can't be considered human sacrifice, cause I'm sure there's Bible verse or two about that! Hee. Actually, they love to be "workers" and get confirmation service credits, so it's definitely a win-win.