The height of an alien mortal is inversely proportional to the home planet's gravity, and thus the mass of the alien's world. Larger planets with more mass have more gravitation pull and would naturally favor short mortals with lower centers of gravity. Conversely, smaller planets with less mass would allow taller beings to evolve.
Eyes are generally adapted to the ambient light of an evolutionary world. Larger eyes facilitate low light conditions or nocturnal life. Round iris configurations are generally good at controlling light in bright conditions, like we have on Earth. Irises with slits, like those found in cats and some reptiles, are designed to reduce incoming light more efficiently, protecting extremely sensitive receptors that work optimally in the dark.
It is generally assumed that the larger head size is indicative of a larger brain. Recent studies of humans has validated this.
-- Paper 49, Urantia Papers