^^Agreed, the steers will vary in size depending on where you go. I don't team rope, but my Dad and Brother used to. I'd see cattle ranging from 350 pounds (those wiry, gangly longhorn crosses) to almost 800. I never measured height but they would sometimes vary by several inches...even in the same pen. The first steer in the chute might be 5-6 inches shorter than the one directly behind it LOL.
I think what I would likely do is make your dummy where you can adjust the height on it to get you used to roping all different heights from about 24 inches to about 40. It's unlikely you'll see steers with that big of a difference, but knowing that you can rope it would surely help your overall ability.
Yea sometimes heading you have to adjust your swing in order to rope right, but on the heeling end you should have the same angle. Me and my brothers just measured what a hale bail was made a dummy our of 2x4's and took some rope and put a steer head on it. I like the height of that verses some of the taller ones I've roped on.
Are you using the dummy from the ground, or the back of a horse? If mostly from the ground then 20-24", if from horseback, then 36"-42" should get'r done. A hay bale works great if roping from the ground.