First get the pony sanding straight and level, then measure the heart girth (H) using a tape measure that runs tightly around the pony lying at the base of the withers and as close to the front legs as possible.
Next measure the length from point of shoulder to point of buttocks (L)
Remember that a pony that is under or over its ideal weight cannot carry as much weight as one that is fit. Referees will be looking out for this. When a pony is at ideal weight its ribs can easily be felt but cannot be seen when it is standing straight, but will possibly be seen when it turns.
For guidance, a rider may be considered unsuitable for their pony by being too tall if, when standing by the pony the riders armpit is more than 10cm above the pony’s withers