Howdy,
5/16" or 8mm would be strong enough but remember... the brake rod goes into compression when the pedal is pushed, so make sure the linkage rod size is sufficient to prevent buckling under load. Also, pay attention to the dissimilar metals corrosion potential of having stainless parts screwed into aluminium. Use some sort of joint compound (in Australia we use 'Tef-Gel') to eliminate this risk.
With all due respect, I made an 8mm adjustable rod and it bowed and bent under hard braking. I had even added a 2" thread coupler in the center to alter the stress moment vs no coupler. Albeit, the rear tire would be leaving big black streaks before the rod could bow, but the tire shouldn't lock up completely due to ABS.
I cut a position one rod about 2 inches in front of the clevis weld, drilled and tapped 3/8- 16 threaded rod, about 1.5 inches on both rods, and added an unnecessary nut to lock the threaded rod in position against the end of the tubing. Dry fit the rod length with your boot under the pedal if you ride that way. It did NOT bow or bend with me exerting 70 pounds of force. It's up to each individual if they consider an adjustable rod safe. LaMonster's rods are better, the same as the stock design, and they will never bow or break. They are just a bit shorter than the factory rods, which moves the pedal closer to the floorboard, but the pedal angle is not adjustable.
I'd highly recommend non-adjustable rod if anyone one has ANY safety concern whatsoever with an adjustable 3/8- 16 threaded adjustable rod. Can Am Engineers designed a very robust and fault free rod by making it completely solid with welded clevis ends.
But I like to tinker (I am fairly sure a few others on this site are tinkerers too, lol) and I wanted to see what I could do for literally 30 cents of thread rod and one unnecessary nut.
If you need to buy a tap and handle plus a 5/16 tap drill, then the price escalated by a huge percentage vs 30 cents but only to about $12. But you then have the tools to use again, so it's not really fair to include all the tool costs, perhaps.