Visual angle. An image on the retina is measured in terms of visual angle. Visual angle depends on the size of an object and its distance from the eye. This eye is drawn at actual size, so the visual angle is correct for an object at this size and distance. Of course, we do not usually view objects at such close range; visual angles for objects seen in the real world are often fractions of a degree.
Drag the arrow to change the distance; drag the arrowhead to resize it.