Je voudrais essayer cette robe dans une taille plus grande.

Pronunciation: zhuh voo-dreh eh-say-yeh set rohb dahn oon tye-yuh pluh grahn-duh.

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

This polite request is essential when shopping for clothes in French-speaking stores, and using the conditional "je voudrais" shows politeness and respect to shop staff.