To choose the correct size for the Colmi Smartring, it is important to compare the measured circumference with the available product measurements.

- The 7 cm circumference you measured corresponds to a specific ring size.
- Usually, manufacturers provide a table of measurements to help you choose the right size. You can consult the 'Description' or 'Specifications' section of the product page to find this table.
- If your 7 cm circumference does not correspond exactly to one of the standard sizes, choose the nearest size that is not too tight, as the ring materials may be slightly flexible.

If you cannot find the specific information on the product page, you can also contact the retailer's customer service for further assistance.