Made to measure belts can be created to fit you, all you need is to check your measurement. Your trouser size may be 36" but check, they may be 40". Two pairs of trousers are never the same size - try it - measure your waist and check the size of your trousers. A bespoke belt means you get a belt that fits!
Use a tape measure! Put over the type of clothing, as if the tape were the belt, alternatively, measure the belt you have with the tape measure. Lay out the belt and measure the distance from the hole at which you wear it to the very end of the buckle.
Windsor Saddlers will post a tape measure if you like, just ask.