Size 5 is the extra-large option in this series, so it is meant for people who need a roomier fit. The support only works as intended if the sleeve is in contact with the skin and stays in place during movement. If it is too loose, it can slide or bunch behind the knee; if it is too tight, it can become uncomfortable and make bending difficult.
Start by measuring the circumference of your knee with a flexible measuring tape at the centre of the kneecap while standing. Then compare that number with the manufacturer's size chart for this model. Because sizes are not the same between manufacturers, do not rely on your usual clothing size. The 23 cm height is another point to consider: the sleeve covers a larger area above and below the knee than shorter models, so it needs enough room on your thigh and calf to sit smoothly without rolling.
When you try the sleeve, it should pull on with some resistance and feel firm but comfortable. You should be able to bend and extend the leg fully without the edges digging in. If you have swelling, or if you are unsure about fit, get advice before choosing a size.