That is a great question. Many of us have seen the Moon near the horizon* and felt sure that it looked larger than later in the evening, as it rises. Try this exercise for fun:

Get a stiff piece of paper (like a 3-in x 5-in card) and a pencil or pen. When the Moon rises near the horizon, hold the card at arm's length, lock your elbow, and hold your arm straight. Move your arm and hand so that the card goes across the Moon's widest part, its diameter. With your other hand and the pencil, mark the edges of the Moon, where they cross the card. Mark carefully, as the shape will be only a few millimeters wide. Now, wait a few hours.

You can use the same card that you marked a few hours ago, but it is quite interesting to get a second card and repeat the measurement independently. Mark the Moon's diameter along the edge of a card. Hold the two cards edge to edge: The Moon will be the same size!

Studies have been carried out with disks of different diameters mounted so that they can slide on tracks, to determine why the Moon looks smaller when there is nothing behind it but blue sky or stars. No one is sure, but apparently the presence of familiar objects along the horizon throws us off.

If you find yourself among trees, try this one too. Observe the Moon with tree branches in the foreground. Now, walk a few paces to your right or left so that the Moon is hanging in clear sky with no branches in the way. It immediately looks smaller! You know it hasn't changed size, but it looks like it has.

Our brains compare the Moon to buildings and trees, and figure that it must be huge! Well, it is big; it's 3,476 kilometers (nearly 2,150 miles) in diameter. That's about a quarter the diameter of Earth. Although it's a long way off--384, 400 kilometers (almost 238,900 miles) away--when it's low near the horizon, it looks like you could reach out and touch it. Of course if you claimed you really could touch it, I might think you're loony, or Moon-y. Keep looking up.