The difference between 'sana' and 'size'
In this sentence, it would be right to use both 'sana' and 'size'. However, they mean different things.
Sana is a singular 2nd person dative pronoun. On the other hand, size is a plural dative pronoun, or it is used when one is being formal.
In this case, 'çocuk sana gazeteyi okur' would mean: 'the child reads you (singular) the newspaper'. Whereas, 'çocuk size gazeteyi okur' would mean: 'the child reads you (plural/formal) the newspaper'.
I hope this helps.