The first factor of the 4C's that most people learn about is carat - the weight of the diamond, which is also the best indication of a diamond's size.
Let's take a look at carat in detail, to understand how it affects the price.
A carat is the standard unit of weight for diamonds. One carat equals 1/5 of a gram, or .007 of an ounce. In other words... there are 5 carats in 1 gram and 142 carats in 1 ounce.
You also may have heard about "points". This does not refer to the number facets on a diamond, but rather to its weight. Just like one pound is divided into 16 ounces, one carat is divided into 100 points -- so each point is 1/100th of a carat. A "10-point" diamond weighs 1/10th of a carat, and a 50-point stone weighs one-half carat.
How it affects the price?
Carat weight has a great deal of influence on the price of a diamond. All other factors being equal, as weight increases, rarity and value of a diamond will also increase. For instance, one diamond weighing 2 carats will always cost much more than two diamonds of the same quality weighing 1 carat each.
So, if size is an important factor for you or the person you are buying for, less emphasis can be placed on the other C's to choose a larger diamond.