Hegar sizes are similar to French sizing. The size number is the circumference of the dilator in mm. So, the diameter is the size/π. (π = 3.14).
Above table is based on 'The quantitative anatomy of the normal child's heart. Rowlatt, UF., Rimoldi, HJA., Lev, M. Pediatr Clin North Am 1963;10:499'.