Hi Kelsie,
Sunspots are located at the regions where the magnetic fields are very strong. The reason for a sunspot to be darker than the quiet solar surface is that the temperature in the region is lower because the strong magnetic fields suppress the motion of the material, kind of freeze the material, if you like. Therefore the size of a sunpot is determined by how large of the cross section is of the strong magnetic field region. Often the strong magnetic field regions are explained by scientists to be twisted fluxropes, then the size of the sunspots will be determined by the size of the fluxropes and the strength of the magnetic field in the fluxropes.
Yan Li