It's too small.

ASSISTANT:Do you like this dress, madam?
LADY:I like the colour very much. It's a lovely dress, but it's too small for me.
ASSISTANT:What about this one? It's a lovely dress. It's very smart. Short skirts are in fashion now. Would you like to try it?
LADY:All right.
LADY:I'm afraid this green dress is too small for me as well. It's smaller than the blue one.
LADY:I don't like the colour either. It doesn't suit me at all. I think the blue dress is prettier.
LADY:Could you show me another blue dress? I want a dress like that one, but it must be my size.
ASSISTANT:I'm afraid I haven't got a larger dress. This is the largest dress in the shop.