1. The Sun is visible to us two minutes before the sunrise and two minutes after the Sunset, because of the bending of the light due to atmospheric refraction. The light from the Sun got refracted by the atmospheric layers and reaches the observer. This causes the observer to think that the light is coming straight from the Sun.
2. The Sun appears bigger in the horizon during Sunrise and Sunset is due to an optical illusion. Actually no size change occurs during Sunrise or Sunset. Still the cause of the illusion is not explained properly.