Carat refers to the weight of a gemstone. It's a unit of measurement used worldwide for diamonds and other gemstones. However, when it comes to Moissanite, the term "carat" is used a bit differently.
Moissanite is less dense than diamond. Therefore, a Moissanite gemstone will be slightly larger than a diamond of the same carat weight. For example, a 1-carat diamond and a 1-carat Moissanite have the same weight, but the Moissanite is physically larger.
To avoid confusion, most retailers list the size of Moissanite gemstones in millimeters along with a diamond equivalent weight (DEW). DEW refers to the size a diamond of the same weight would be. For instance, a 6.5mm round Moissanite weighs about 0.88 carats, but has a DEW of approximately 1 carat because it's similar in size to a 1-carat diamond.
In summary, the carat of a Moissanite is a measure of its weight, not its size. The size of a Moissanite gemstone for a given weight will be slightly larger than a diamond of the same weight due to the differences in density.