When selecting the size of your diamond, keep in mind that carat is a unit of measure that refers to the actual weight of the diamond. The larger the carat, the heavier your diamond will be, not necessarily bigger.

The shape and cut of your diamond also need to be considered to determine its true appearance, as a poorly cut diamond with a larger carat may appear smaller than the same carat weight in a higher-grade cut.

0.2 grams and one diamond carat is divided into 100 points, meaning that 0.25 of a carat is 25 points or ¼ carat.

Diamonds that fall just shy of the popular weights such as 0.5kt, 0.75 ct, 1ct etc are often priced lower on a per-carat basis but are visually very similar looking in size. Consider 0.40, 0.90 or 1.75 carat if you want more value for money when choosing your diamond. 

When choosing the carat size, also consider the length and width of the ring finger, as some diamonds will appear larger on more slender fingers and smaller on wider and shorter fingers. The type of ring setting can also affect the appearance of the diamond, with delicate settings helping the diamond appear larger, while thicker bands can make a diamond appear smaller.

The carat to size ratio of each diamond is unique, however, the chart below is a guideline that can help you determine the typical size.
0.25 CT=4mm
0.5 CT= 5.2mm
0.75 CT= 5.8mm
1CT = 6.5mm
1.25 CT= 7mm
1.5 CT= 7.2mm
2.0 CT= 8mm
3.0 CT= 9.2mm