How do you call someone fat without being rude.

The name tag was intended to go there. If you think you'd look creepy trying to look at their name tag, it's because you were having other kinds of thoughts. If you are really intent on getting a good word in, just call the store and let them know. You are allowed to say "I didn't catch her name" and then it's Que Sera Sera.

Also, have you never had to describe an overweight person before? How did you handle it then?