When Juliet's nurse compares the Count Paris to a "man of wax," she employs a metaphor to compare him to a perfect sculpture of a man. In making this comparison, the nurse emphasizes Paris's physical perfection in addition to his greatness in the world (which she has already mentioned). She seems to suggest that if a sculptor were to create a perfect specimen of a man, using wax as his medium, he would craft him to look just like Paris. The nurse conveys her clear approval of this suitor for Juliet's hand in marriage, repeating Lady Capulet's claim that "Verona's summer hath not such a flower." In addition, then, to his greatness in the world -- he is a count, and related to the prince -- the nurse and Lady Capulet both stress his physical attractiveness (perhaps believing that this might matter more to a young girl of thirteen than the knowledge of his worldly "greatness").
There are actually two literary devices used in this comment from Juliet's Nurse; they are hyperbole and repetition. The nurse uses an extreme exaggeration to help the argument for Paris along when she says that he is the best man in all the world. This helps her to support Lady Capulet in their attempt to convince Juliet to consider his suit for marriage. The repetition of the word "man" helps to put her point across as to the importance of the situation; and, the fact that she isn't using the word "boy" suggests that he is more important than the boys of Juliet's age. Combined, both hyperbole and repetition show the Nurse's excitement for the situation and her support that Juliet considers him.