Stress, size, weight, type of plane. pushrod length/bracing are all factors. A Unlimited Class Fun Fly plane only has a 46" wingspan, a .32 engine, and weights less than 3 pounds, but requires 4-40 rods. My first 1/4 scale, 72", G23, 15 pound Fokker Dr1 flew nicely with 2-56.

The geometry of the pushrod run and the type of pushrod are more important than the size and will dictate what needs to be used.