A simple tape measure is enough. Measure the heart girth just behind the withers, and the body length from the point of the shoulder to the point of the buttock.
Classic veterinary formula: (heart girth² × length) ÷ coefficient, the coefficient varying by size (11,877 for an adult horse, 11,000 for a pony, 12,265 for a yearling, 11,671 for a foal), measurements in centimetres.
Indicative estimate. It does not replace the advice of a veterinarian or professional.