I would agree with these conclusions. 5 cm, +/- and couple mm, seems about right for a normal German vehicle antenna. Also, the antenna guard on the side of my PzKpfw IV Ausf G is about 4.9 cm long.
I do have some references on scout vehicles that quote them using 3 meter antennas. These would scale down to about 8.5 cm.
Enjoy your modeling...