^^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.