Je voudrais essayer cette robe dans une taille plus grande.

Pronunciation: Zhuh voo-dreh es-say-yay set rob dahn oon tye-yuh pluh grond.

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

This is a practical phrase for shopping in French boutiques; use "Je voudrais essayer" (I would like to try on) followed by the item and size to politely request fitting room access.