As with all precious stones, the weight and therefore the size of a diamond is expressed in carats.
One carat is divided into 100 "points" so that a diamond of 25 points is described as quarter of a carat or 0.25 carats. Size is the most obvious factor in determining the value of a diamond but now you know that two diamonds of equal size can have very different values on their quality. However, remember diamonds of high quality can be found in all size ranges. Here are some other examples that show the approximate sizes of the diamonds in question.