Learn Turkish: Senin şehrin küçük mü?

The dictionary word for city is şehir. However, some two-syllable Turkish words drop the vowel in their second syllable when you add a suffix that starts with a vowel. Since the suffix for "your" is -in, the second "i" in şehir disappears, making it şehrin.

If Senin already means "your", why do we add -in to the end of şehir?

In Turkish, possession marks both the owner and the object. You use the possessive word Senin (your) and also attach a possessive suffix -in (your) to the object itself. You can actually drop Senin and just say Şehrin küçük mü?, but you cannot drop the suffix on the noun.

Why is the question particle mü here instead of the usual mi?

The question particle changes to match the vowels of the word right before it, following the rules of 4-way vowel harmony. Because the last vowel in küçük (small) is "ü", the question particle must also use "ü", becoming mü.

Where is the word for "is" in this sentence?

Turkish doesn't use a separate word for "is" in simple descriptive sentences like this one. Simply putting the adjective küçük before the question particle mü is all you need to ask "Is it small?".