Size 6 is intended for a larger body circumference, so it will be the correct choice only if your midsection is in that larger range. The product page does not list the exact centimetre range, so the practical way to check is to take a soft tape measure and measure your abdomen around the widest point, usually at navel height. Stand normally, do not pull the tape tight, and write the number down. Then ask your doctor, physiotherapist or a pharmacy assistant to help you compare that measurement with the size instructions on the package.
Fit matters more than the number. The belt is designed to wrap around the abdomen and give light, even compression. When it is on, it should feel held together without being painful. You should be able to breathe normally, sit without feeling the belt dig in, and move without it sliding. Because the belt contains rigid ribs to keep its shape, an oversized fit can leave the ribs pressing unevenly, while an undersized fit may restrict your breathing or movement. If you feel numbness, tingling or pain while wearing it, remove it and stop using it until you have checked the fit with a specialist.