You typically don't want anything more than 1/8" longer than your lug height, so as your sig says, I'm assuming your Cobra is 1 3/8? If so, stay around 1 1/2.
That being said, I also would add (I ride Cobras as well) that the Cobra is a softer rubber than many other tracks, so I personally don't and wouldn't stud mine. What conditions do you normally ride in? If you generally don't ride beat up, icy trails, studs aren't going to help much? I could be wrong, but I don't seem to see as many people studding anymore.
If you're set on studding, you should pick a pattern that suits your riding, and plan on around 168 studs. Choose your backer plates carefully as well. Some will help prevent pullout better than others.