This is the easiest of factors to determine as is it is a simple weighing process. Diamond weight is measure in carats which is exactly 200 Milligrams (mg) or 1/5th of a gram 0.007055 oz.).
One carat is typically divided up into 1/100ths so a typical gemstone for a ring for example might weigh 0.75 Carats (150 mg). The largest diamond in the world is the Golden Jubilee Diamond weighing an enormous 545.67carats cut (3106.75 carats uncut). It is considered an ugly stone however, as its color is a deep yellow and not deemed that attractive to most. The King of Thailand now owns it and it is said to be incorporated into a new piece of state jewelry at some point.
At the more usually seen end of the carat scale are diamonds ranging from the relatively small at under a ¼ carat, to the larger at perhaps 5 carats or more. An engagement ring for example typically bears a solitaire diamond centerpiece at between 0.5 and 2 carats but obviously the more money you can afford; the larger the weight in carats a diamond can be and can be obtained at up to twenty carats or more for a single stone. The carat weight is an obvious pointer to the value of a stone but the cut/color/clarity are other major factors affecting the price.