I have noticed that the Moon looks considerably larger when near the horizon, than it does when viewed at the zenith. is this an impression of the brain, or some kind of optical illusion?
It’s an optical illusion, which is an impression of the brain, I guess. Here’s one of Cecil’s columns.
There ya go.
It’s really an optical illusion. Phil Plait explains it, and has some links, here.
It’s demonstrably the same size – if you measure its diameter on the horizon (using, say, coins held at arm’s length, or marking it with strips of tape on a window while you stay a fixed distance back), you can verify that it remains the same size. But the illusion is compelling, and has ben treated many times. Besides the aforementioned SD column on it, you can google “Moon Illusion”, or look in books like Minnaert’'s “The Nature of Light and Color in the Open Air”.
Even easier: hold up your hand to block any objects below the moon. It instantly appears to shrink.