If you are unsure about your size, the best option would be to purchase a Size Kit. Please keep in mind that sizes may slightly vary depending on the shape you choose. So if you want to know your size for several shapes , you should purchase one kit per shape.
The second option is to measure your nails with measuring tape. Disclaimer, since different shape sizes may vary, measuring to find your size for multiple shapes may not give you the best fit for each shape.