The interior surface is between 9m² and 12m² in general. Below 2.50m (98 inches) a ceiling is considered dangerous. The height must therefore allow to place the openings above the horse (minimum 1.80 m). So for a window in a box, I would say what should be about 1.30m (51 inches) from the ground. That way all safety rules are respected. This prevents currents of air from passing, when the horse sleeps head down or lying down.
I hope this helps :)