I'm making kitty play equipment too. Is there a rule of thumb or ideal for height off the ground for boxes? Or spacing between platforms?
They'll adapt to anything, so there's no hard-and-fast rule, cats can jump amazing distances to get to where they want to go. One thing you can do if you want some sort of guide is take commercially-made stuff, presumably in China, and scale it up by about 50% to get realistic sizes for things.
And then when you're done with your $300-worth-of-materials-and-labour creation you'll find the kitty prefers to sleep in the cardboard box the brackets came in.