"You are small."
Lets see. We have the sentence 'You are small'. First of all 'Küçüktür.' is not used for the pronoun 'you'. Since 2nd person singular (you) is given we use 'sün' in this case right after the adjective which can be translated as 'are' in 'You ARE small.' -> 'Sen 'küçükSÜN.' For 3rd person singular and 3rd person plural you can use 'kücüktür' though. Therefore if you want to say 'He/She/It is small.' you say 'O kücüktür'. And 'They are small.' would be 'Onlar kücüktür. However you can use 'kücük' alone aswell in both 3rd person plural and singular. 'Tür' would'nt actually be necessary in order to understand the sentence. It would be totally fine to just say 'O küçük.' and 'Onlar kücük.'(The translation still ends up being the same). Its only possible with those two cases though and I'd say its more directly. You can also leave out the pronoun if its already obvious on what you are talking about but the meaning of 'tür' can/will most likely change e.g -Pantolon bana yakışır mı? (Would the pants look good on me?) -'Küçüktür.' -> "They are (probably) small.