Your belt size is not your waist or pant size; you need to allow for the material you are wearing under your belt. Additionally, if you are wearing an IWB holster or other gear it will artificially increase your waist size.
If you have access to a cloth measuring tape, run it through your belt loops like a belt. Make sure you are wearing the same setup that you will utilize your Boxer belt with (e.g. IWB holster or other gear). Pull the measuring tape as tight as you’d like to wear your belt and note that measurement.
If you don’t have access to a cloth measuring tape, no worries. Find a belt that fits you properly, lay the belt flat and use a standard measuring tape to measure from the buckle base to the belt hole that is most comfortable.