Choosing a tenkara rod
Take 3.3 m to 4.0 m unless you know otherwise. That band covers most of the range and suits most mountain streams. Shorter rods handle tight, overgrown water; longer ones out to 4.8 m give more reach on open runs, at the cost of being harder to work under trees.
Choose flex for how you want to cast. A 7:3 rod bends in the top third and feels crisp, which suits a level line and an experienced caster. A 5:5 bends through the middle, loads more slowly and is more forgiving. Expect to find this in the model name rather than in a spec table.
Decide on line type early, because it changes the rod you want. Level line is a single constant-diameter fluorocarbon and is the technical choice, light enough to hold off the water almost entirely. A furled or tapered line turns over more easily and is kinder to learn on.
Match tippet to the rod, not to habit. Most rods here specify 0.8号 to 1.2号, about three to five pounds. With no reel and no drag, the tippet and the rod's flex are the entire shock-absorbing system.
Check the collapsed length if portability is the reason you are here. These pack to 53 cm to 60 cm, and several ranges are built specifically around that, which is the whole practical case for the method.