What size do you need?
¿Qué talla necesitas? You don't use te necesitas, right?
2 Answers
"Que talla necesitas" is correct, with "c" on second syllable.
You need to think who is the speaker and who the listener. For example: in "Te necesito" 'You' is what I need, so there is a complement there (te). But in ¿Qué necesitas?, we alrady have "qué" (what). Also, if you look at the meaning, in the first example we know what is needed (because it's written there - "TE" necesito), but in the second one, we are asking about it, as we don't know.
Sorry, it sounds a bit funny to write that explanation, rather than telling it! But I hope you understand (more or less) what I tried to say.
Un saludo.