The problem is: Que talla usar/usara/usaro Pedro? Does usar get affected by the subject or any ol noun?

The verb must agree with the subject, Pedro. However, none of the verb forms there (usar/usara/usaro) will work. Could you possibly miss an accent mark? I ask that because the future (usará) definitely works

What cut will Peter use? = ¿Qué talla usará Pedro?

Sra