How do you properly weigh up to determine whether your beagle is classed as 13 or 15 inch stage ?

There is a measure tool called a wicket. It measures dogs to the withers (shoulders).

Put the dog in a standing position. Place a tape measure on the floor right at the front paw and judge straight up to the withers. (shoulder) It is the highest part, right aft the neck. You will be able to feel the dog's shoulder blades.

it is measured from from floor to shoulders on the front legs