In comparing sizes, the usual method is to figure out how many times larger or smaller one object is than another.

For example, if we wanted to find out the relative size of the Earth and Moon, we could compare how many times bigger the Earth's radius is than the Moon's radius.

The Earth's radius is about 6,400 (=6.4x10³) km. The Moon's radius is about 1,700 (=1.7x10³) km. To find their relative size we simply divide the bigger value by the smaller one. Thus, the Earth is 6.4x10³ km/.1.7x10³ = 3.8 (approximately) times larger in radius than the Moon.

Note: In making such comparisons it doesn't matter if we use the radius or the diameter. Because the diameter is just twice the radius, the factor of two will cancel out when you divide one by the other.