The size of a diamond is measured in carat-weight. One carat is divided into 100 ‘points’, so that a diamond of 75 points, weighs 0.75 of a carat.
While carat-weight is the most obvious factor when assessing the value of a diamond, two diamonds of equal size can have very different values, depending on their respective cut, clarity or colour. Bigger isn't necessarily better - diamonds of high quality can be found in all size ranges. However, one diamond of one carat is normally worth more than two diamonds of 0.50ct. each.