Je voudrais essayer cette robe dans une taille plus grande.

Pronunciation: zhuh voo-dreh eh-say-yay set robe dahn oon tah-yuh pluh grahnd

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

This polite conditional phrase is essential for shopping in boutiques; French shops typically have fitting rooms (les cabines d'essayage) where you can ask staff for different sizes.