As with all precious gems, the weight, and therefore the size of the diamond, is expressed in carats.  The carat originated as a natural unit of weight:  the seeds of the carob tree.  Diamonds were traditionally weighed against these seeds, however, the system was later standardized and one carat was fixed at 200 milligrams (1/5 of a gram). 

One carat is divided into 100 “points” so that a diamond of 25 points is described as a quarter of a carat or 0.25 carats.  Size is the most obvious factor in determining the value of a diamond, but two diamonds of equal size can have very unequal prices, depending on the other characteristics of cut, color and clarity.