Why does the sun appear larger at dawn and at sunset?

Anything which is part of the normal landscape or sky (trees, hills, birds,clouds, planes) usually looks smaller when they're near the horizon. The reason for this is simple: stuff near the horizon is generally more distant. Just take trees in the landscape, or a road disappearing in the distance as an example. Because of this our brain knows and expects that objects at or near the horizon appear particularly small compared to their true life size.