Je voudrais essayer cette robe dans une taille plus petite.

Pronunciation: zhuh voo-dreh eh-say-yay set robe dahns oon tye pluh puh-teet

Meaning: I would like to try this dress in a smaller size.

This is a practical phrase used in French boutiques; remember that French sizes run smaller than American sizes, so French shoppers often ask for larger sizes when shopping internationally.