Once you know your Rear Center, often listed as Chain Stay Length, it's easy to figure out what you need using Gates's calculator. If you have a Spot bike, you may want to check what your current sprocket sizes and belt lengths are before continuing. If your bike is a current model, you can go to the respective product page, scroll down, and click Component Specs. You should see the stock Gates components that came on your bike. Also, if you click on Geometry Specs, you can see the Rear Center or Chain Stay Length. For Spot bikes, be sure to note the Min and Max Rear Center lengths. As long as you stay within those, you can choose Gates components as you wish.
Once your homework is done, go to the Gates calculator here: https://www.gatescarbondrive.com/Apps/DriveCalculator/index.html