Hi Larry; Welcome to Permies:
I'm sure you can find free plans out there somewhere. I don't have an exact address for you.
I suggest, you start looking at the posts here at permies. Lots are start to finish builds.
You can figure out the dimensions easily enough. We call it rocket science as a joke. Although proper construction methods and dimensions are critical to it working well and working safely!
But, seriously buying the RMH builders guide is the best way to learn proper terminology, dimension's, what has been tried and worked and what not to do. It is worth every penny.
People wing it on their own all the time. A few succeed … most have issues and end up buying the book and asking what is wrong here... A few Give Up and say it doesn't work... there's always a few "special" people in every crowd!
Rocket stoves are built by boy scouts and ice fishermen. Rocket mass heaters , Batch boxes , J tubes, brick bells, piped mass, those are built by rocket scientist's.
Grab a lab coat and become a rocket scientist !