There are so many ways to measure the size of the Moon: diameter, volume, surface area, and mass. Let’s look at how big the Moon is in every possible way.
The diameter of the Moon is 3,474 km. Need some comparison? The diameter of the Earth is 12,742 km across. That means that the Moon size is roughly 1/4th the size of the Earth.
The volume of the Moon is 2.195 x 1010 km3. That sounds like a huge number, but it’s actually only 2% the volume of the Earth. In other words, you could fit 50 moons inside the volume of the Earth.
The surface area of the Moon is 37.9 million square kilometers. How big is that? Just for comparison, the surface area of Asia is 44.4 million kilometers. If you could unwrap the Moon, it wouldn’t quite fit inside Asia.
The mass of the Moon is 7.347 x 1022 kg. Once again, that sounds like a big number, but the mass of the Moon is only 1.2% the mass of the Earth. If you had a great big scale, and put the Earth on one side, you’d need a pile of moons stacked 81 high to match the mass of the Earth.
So now you know the size of the Moon.
You can listen to a very interesting podcast about the formation of the Moon from Astronomy Cast, Episode 17: Where Did the Moon Come From?