For a round brilliant cut diamond, the size or diameter can vary depending on the carat weight and the specific dimensions of the stone. Here’s a general guideline:

Diameter (mm): The diameter of a round brilliant cut diamond of 2 carats can vary, but as a rough estimate, it might be around 8.1 mm to 8.2 mm.
Depth: The depth of the diamond (from top to bottom) contributes to its overall appearance and brilliance.
Carat Weight: A 2-carat round brilliant cut diamond typically weighs around 0.40 grams.
If you have more specific requirements or need detailed dimensions for a particular purpose, it’s best to consult with a jeweler or diamond expert who can provide precise measurements based on your needs.