Learn French: Quelle taille est-ce que vous cherchez ?

Quelle is an interrogative adjective (meaning 'which' or 'what'), and it has to agree with the noun it describes. Because taille is feminine and singular, we use the feminine singular form Quelle instead of the masculine Quel.

Why is Quelle taille placed before est-ce que?

When you use est-ce que to ask a question, any question words or phrases, like Quelle taille (what size), must go right before it. The structure is always: [Question phrase] + est-ce que + [subject] + [verb].

Where is the word for 'for' in the French sentence?

You don't need one! The French verb chercher means 'to look for' all by itself. So, vous cherchez translates perfectly to 'you are looking for'. Adding a preposition like pour here would be a grammatical mistake.

Does taille also mean shoe size?

Usually, taille refers to clothing size or a person's physical height. If you are shopping for shoes and want to ask about shoe size, you would use a different feminine noun: pointure. The question would then be Quelle pointure est-ce que vous cherchez ?