Je voudrais essayer cette robe dans une taille plus grande.

Pronunciation: Zhuh voo-dreh eh-say-yay seht robe dahns oon tye plus grahnd.

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

This polite request is essential when shopping in French boutiques, where staff will often ask "Vous désirez essayer?" (Would you like to try it on?) and knowing how to specify your size makes the experience smoother.