TB 746-93-1 does not specify the size of the Unit (Bumper) Markings, only that they be the largest size practical, no larger than the Registration (Hood) Numbers (which are "3") and that they be applied preferably with vinyl lettering or gasoline-soluable paint.
I think I read somewhere that the letters should be half the height of the bumper and must be centered vertically on the bumper. In the end, these markings were applied at unit level at the discretion of the unit commander and therefore can be just about anything. There is no ""proper" way to do it. Just do whatever you think looks good.