I'm sorry to disagree but this case it's an example of an indirect question, a more polite way to make a question. the question as you propose it would traduce Voçê sabe o seu tamanho? so it's different what in the example wants to express.
We're talking about the size the person might wear, so his size, or her size. You could specify and ask about the size of the dress or the coat. You could ask 'what size is it?', but how you phrased it doesn't sound right at all.