Question

give a method to determine the size of a moon

Answer 1

measure when we look at the sky are angular sizes and angular distances. If you divide the whole celestial sphere in 360degrees just as you would do for a circle, you can talk about the angular size of objects or their angular distance (the angle that appear to separate the two objects on the celestial sphere). When we calculate these angles for the Sun and the Moon, we surprisingly find almost the same result : 1/2 a degree. (this is just a coincidence, and fortunately for us it is the reason why we can get really nice solar eclipses).