It will depend on what engine you have. Most cam and/or rocker arm manufacturers have studs and guide plates. Summit carries quite a few of them, go to their website, enter year, make, model and engine.
the rocker stud od depends on the spring rate and/or rpm . . if you run a mild to moderate cam then 3/8" are fine but the 7/16" aren't much more and the labor is the same but there are may be a few more rocker arm choices in 3/8" so i might find the rockers i want to use first . . scorpions "race" series are the best bang for the buck.
you will need to properly set your rocker arm geometry.