The weight or size of a diamond is measured in carats.

A carat is 0.2 grams and there are 100 points (or 200 milligrams) per carat.

To give you a general idea about size, the table below shows the
approximate relation between the diameter of the top of the stone and the carat weight.

"Fewer than 1% of Women will ever own a diamond of a carat or more."
Source: The Diamond Trading Company (2006)