Diamond size is measured in weight known as Carats. You can download a very useful guide that will list the actual sizes of all diamond shapes by clicking here.
What is a Diamond Carat?
According to Wikipedia, the term diamond carat comes from the Greek word meaning “fruit of the carob tree.” In ancient times, the seeds of the carob pod were used as standards for weight measurements, since the size of their seeds were almost always identical. This provided a natural standard of measurement.
In in 1907 at the Fourth General Conference on Weights and Measures, it was agreed that a modern diamond carat should equal precisely 200 milligrams. This is technically known as a metric carat, and is the standard diamond caratweight that is used universally today.
To the right is an example displaying the difference between a well-cut diamond and a poorly cut diamond.