I hope that's for the 1/2-A models.
The question is one of those whose best answer is "it depends". I tend to use 5mm or better OD arrow shafts for big models, and 3mm OD tube for my 1/2-A models. But I had a Fancherized Sig Twister for a very productive few months (before I stupidly crashed it) that worked great with 4mm OD rods.
I decide whether a rod is stiff enough by pressing the ends together -- if the rod bows enough that I can feel it get springy, then I use something bigger.