Step 1: figure out what size it needs to be.
Here’s my Cyr Wheel sketch for my size. I’m 5’6″, and if I add about a fist height (1.5″ to 2″), that gives me 68″ inner diameter. I’m using 1.5″ aluminum tubing (that is 1/8″ thick), and 1/8″ thick PVC tubing for a skin. The tubing will slightly reduce the inner diameter (by 1/4″), but I’ll ignore that. That means I want a 34″ inner radius, and need at least an 18′ piece of metal to make it work. Each segment would be ~72 3/4″ long for a 3 piece cyr.