The Universal Tip Cap is designed to fit most Tenkara rods. The “normal” size will fit most of your Tenkara rods, for wider rods like Keiryu you might consider the “large” size.
Normal Size can fit up to 9/16 inch (14 mm) and the Large Size can fit up to 11/16 inch (18mm), it is best to choose the smallest size that will fit your rod.
I love the Universal Tip Cap because it is more functional than the traditional rod tip plugs. They hold very securely and do not slide off on accident. You can keep your line attached to the rod with out it getting pinched by a rod plug. And most people find them harder to loose track of.
I use the Universal Rod Tip Caps on most of my rod and I think you will love them