I need to build a Rifle Frame for Kokohead. It looks like there is a height range between 48" to 80" high. What would be a suitable height for 100yrds shooting 5.56? I have a SUV so a 48" high frame would work perfectly for transportation.
I have only shot military pop targets or used ranges that had their own targets so I never thought about the "science" of target frame size. Assuming it is a science.
My truck bed is slightly over 5', so I use about 60" height. I leave space at the top for steel plates and connect them with chains on the front of the frame so spatter is deflected forward of the wood. Otherwise you'll quickly destroy the legs. Ask me how I know ....
Brand new frame after one range visit:


Notice the chains were attached to the horizontal bar, and that was behind the verticals. I basically turned the frame around so the horizontal piece was forward of the legs, and attached the plates in front of the horizontal. No more frame damage from bullet spatter.
As long as the legs are spaced apart properly to fit the frame holes, the legs are tapered to a round-ish shape (square lumber in a round pipe in the ground), and the rules for shooting steel are followed, you'll be fine. I've seen some creative targets out there. Some super tall or short, and some super wide. Just depends on your needs and vehicle dimensions, I guess.
I believe 50 yds is okay for steel plates using a rimfire rifle (.22 LR, .17 HMR). Centerfire are only allowed at 100 yds. Shotguns on paper targets must use slugs only at 50 or 100, but there are some 25 yds holes at one end for shotguns. For steel, shotguns are included in Centerfire rules.
The lumber dimensions are used so they are close to the size of the pipes in the ground. Technically, you can engineer a frame with the right size and spaced-apart legs to fit the pipes, then attach lumber that's of different dimensions.
Here's an example of a frame that's part wooden/part steel. FabTek Engineering made/makes some steel frames that can be used for just reactive (steel) targets, or use hooks to hold a wooden frame above it -- basically 2 targets in one.
https://www.fabtekengineering.com/koko-head-systems