Sunspots are places where the magnetic field beneath the solar surface (the photosphere) "punctures" the surface and continues into the outer solar atmosphere (the corona). So the size of the sunspot depends on the magnetic field: If the magnetic field is strong but concentrated, the sunspot could be quite small. If the magnetic field is more spread out, the sunspot will be larger.
Many sunspots are as big as the Earth's area, and large ones can be 10 times or even up to almost 50 times as large as the Earth. I'm not sure which one is the largest so far, but this website has some interesting information about some notably large sunspots: www.spaceweather.com/sunspots/history.html
And sunspots are grouped together in "active regions," which are regions that have very complicated magnetic fields. These sunspot groups are the places where most solar flares come from!