Hi Beacky,
This is a very good question and we do not really have the complete answer yet. Maybe you will be the one to finally answer this question! We do know that it relates to magnetic fields on the Sun and how the Sun rotates. The Sun is made up of charged particles. When positive and negative charges move opposite to one another they create electricity. Electrical current creates a magnetic field. Charges are affected by magnetic fields so they move differently because of the magnetic field. Then the different way that the charged particles move creates a different electrical current and so the magnetic fields change...this goes on and on from charged particles moving to magnetic fields changing. This "feedback" gives us magnetic "loops" popping out of the Sun and the size of these regions are about the size of the sunspots one observes. The magnetic properties of sunspots also determine how long the sunspot will last.
Laura