"You are small."
The "sün" is like "are" in english .. well it's not an actual "are".. like saying (sen küçük) means (you small) and (sen küçüksün)d mean ( you are small) something like this :p .. i really suck at explaining sorry!!
Siz is plural in Turkish. Sen is singular. In English, plural and singular are the same. You = sen and siz. But in Turkish, different.. I hope i can help you.. :)
The suffix should match the person. -sün for you (sen) and -süz for you (siz)
I don't think that Duolingo won't accept it but sometimes you can tell a verb without " You, Me..." For example this can be told like this: Istead of using "Sen", you can write " Küçüksün" only.
We can also say ''ufak'' which means ''little''. So, ''Sen ufaksın'' also should be acceptable.
I think the reason that is wrong is Siz refers to more than one person whereas küçüksün is the singular 2nd person (sen). So "Siz küçüksunuz" or "Sen küçüksün" should be acceptable.
Siz is plural in Turkish. Sen is singular. In English, plural and singular are the same. You = you and you. But in Turkish, different :) I hope i can help you.. :)
I think siz küçüksün is also write ..beacuse earlier comments i read that siz refers to more formal way
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.