I wrote my success story about exactly that. Buying new clothes that fit well is fun now, but definitely a learning experience. I still think I'm bigger than I actually am after three years.

In JC Penny one time a sales associate asked if she could help me. I was looking for a black "pencil" skirt. She showed me where they were and I picked up a size 8 to take to the fitting room. She said "is that the size you want?". I was mortified and said "Should I take a larger size?". She gave me a quizzical look and said "No, smaller". Sure enough she was right, the smaller skirt fit. Another time I was having difficulty finding jeans I liked in a store. I asked a sales lady, who in my opinion was really skinny, for help. She said "You're like me, you need to go downstairs to the juniors department. You won't find anything to fit you on this floor". Whaaaaat? This not knowing what you look like is psychologically weird, but a nice problem to have.