The vertebral heart score (VHS) expresses heart size in vertebral lengths, so it works for any size of dog. Click the six points in order.
Method (Buchanan & Bücheler 1995, the original VHS paper): long axis from the ventral border of the left main-stem bronchus at the carina to the cardiac apex; short axis at the widest point, perpendicular to it; both summed in vertebral lengths counted from the cranial edge of T4. Use a right lateral view. Normal dogs average about 9.7 vertebrae, with roughly ±1 covering two standard deviations; cats about 7.5 ± 0.3. Breed-specific ranges differ, and breathing or the cardiac cycle can change the score by up to one vertebra (Thrall, Textbook of Veterinary Diagnostic Radiology, 8th ed., ch. 35, p. 703).