I would suggest at least 24 inches long... 30 would be better.
I made my current dogs by cutting shorter ones and welding them onto rectangular tube stock.. I wish I had made them longer. I had some dogs years ago that I liked a lot. the tips on those dogs where wider at the ends and one end was parralel to the dog, the other end was perpendicular. The pyramidal shaped tips are good at all angles but they don't hold as well and the wider tips.