We investigate the initiation sizes of earthquakes over a wide range of moderate to large events. Using the model of Sato and Kanamori, which is based on Griffith crack theory for a circular fault, we estimate the initial crack sizes of earthquakes, using high-quality data recorded at close hypocentral distances. Taking into account the local attenuation and the dynamic stress drop of the earthquakes, we estimate trigger factors and initial crack radii for 35 events in Japan and Taiwan with magnitudes of 3.5 to 7.9. Our results show that all of the earthquakes can be explained with a small trigger factor and initial crack sizes of 12--96 m. For the large range of earthquake sizes, this is a small range of initial crack sizes. There does not appear to be a clear scaling relationship between initial crack size and final earthquake size. Instead, our results show that all the moderate to large earthquakes initiate with initial crack radii on the order of tens of meters.