A diamonds size is judged by its weight which is measured in carats.
The word CARAT comes from the word carob which is the seed that in ancient times was used to balance scales.
These tiny seeds appeared so similar in size and in weight to each other that they were used as a method of measurement. One carat is equal to 200 milligrams.
Logically, the size of a diamond does have a definite influence on its value. Many couples though choose a diamond of smaller weight due to financial restraints or simple preference.