I don't have experience with big models, but I can tell you that the limit for long push rods is the pushing force.
The reason is that when pushing, the rod acts like a slender column under load, and the possible failure is always due to buckling rather than breaking under stress.

Hence, a 2-56 threaded rod that is properly supported along its length is safer and keeps the control surface more firmly in position than a 4-40 threaded rod that is not.

That is the beauty of the pull-pull systems: all the elements work under tension (which is self-aligning); hence, they can be made smaller in diameter.