Every company that makes knee braces has their own sizing chart and range of sizes. Whatever knee brace you are going to use, look up the company online and do a search for their size chart. There are usually links to size charts on web pages where you buy knee braces. So, if you have found a knee brace that you want to buy online, simply look at the web page for this type of link. In general, knee braces come in small, medium, large, and several extra large sizes. Your knee measurements can help you find the right size. Find your measurements on the chart and move across the chart to identify your size. Some sizing charts only use the circumference of the knee, while others require several measurements to narrow down your correct size. Sometimes with your specific measurements, a sizing chart says you could pick between 2 sizes. It's almost always a good idea to pick the larger of the 2 options, as knee braces that are on the large side can be strapped down but knee braces that are too small may not even fit at all or they may cut off circulation. This is especially important if you have large thighs or calves, as the larger brace will likely fit your whole leg better.

Summary:
Look up the size chart for the knee brace you want to use. Pick the size that corresponds with your measurements. Pick a larger size if your measurements fall between 2 sizes.