Turn off the telescope's motor drive (if any). Count the number of seconds it takes for the sunspot to drift past crosshairs or any speck of dust that is visible in your eyepiece. The number of seconds equals the spot's breadth in Earth diameters.
The method is approximate. If the spot is near the central meridian, its size in "Eaths" will be overestimated by about 20 percent. If it is three-quarters of the way from the meridian to the east or west limb, its size will be underestimated by a similar amount (owing to foreshortening).
— Roger W. Sinnott