Quick math: When viewed from the moon, the earth's diameter subtends an angle of about 0.033 radians. On the other hand, if the photo was taken from an orbital

Quick math: When viewed from the moon, the earth's diameter subtends an angle of about 0.033 radians. On the other hand, if the photo was taken from an orbital altitude of 100km, then the lunar horizon is about 600km away from the viewpoint (using Pythagoras' theorem). So at the horizon, a crater the same apparent size as the earth would be roughly 0.033 * 600 = 20 km across.