Yes, there is actually a size chart to determine or measure the size of rhinestones. Rhinestones are sold per size, preceded or followed by an SS number (SS stands for Stone Size). There is no connection with millimeters in this number, in the sense that 6SS or SS6 stones are NOT 6 mm in diameter! But there is a size chart in which the dimensioning is explained, so that you can more easily figure out which size of rhinestones to choose.
So in general, the higher the SS number, the larger the diameter of the stone, and the more bling it will have. Conversely, the smaller the SS number, the smaller the diameter of the crystal and the more luster it will have.