What proportions/dimensions/ratios needed for a J-style firebox cook stove?

Hi everyone
I have read through How to Build A Rocket Mass Heater.
In it, if I didn't misunderstand it, it recommends a 6-8 inch diameter J Style firebox. It also mentions that the heat riser should be about 3x or more of the length of the fuel feed, and about 2x the length of the burn tunnel. So a 16 inch fuel feed would have a 48 inch heat riser.

My question is, are these proportions/sizes/dimensions good to se the the J style firebox itself as a cookstove? If I place some sort of grate/pot stand on top of my 48 inch riser and then place a frying pan on top of it, will the heat coming out of it be enough to fry with? Will actual fire be shooting up out of the riser (just like those metal rocket stoves I see on youtube, whose dimensions I cannot ascertain) or will it just be exhaust?

I haven't had time to prototype yet since I am looking for affordable places to get firebricks.

Thank you very much for anyone who can share their experience.

I built one a few weeks back, it gets very hot!

https://permies.com/t/86886/Rocket-stove-hot-plate

There's no place for steel in a rocket mass heater but for cooking, it will get you there the fastest in a rocket cooker.  You just have to consider it a replaceable item (in your riser).

Hopefully the OP has already built his stove and discovered that, yes, a properly designed 6" J-tube rocket stove will absolutely get hot enough to cook/fry food!

As I built the various rocket stoves (mostly failures) I have made, I came to realize that the dimensions/ratios really do matter and how well sealed/insulated the core/riser are is critical.  Dry-stacking is fine to see how the pieces will fit together, but until you seal it up and dry everything out, you don't really know how it will run or how hot it will get.

One additional lesson learned is that the amount of what I call "stand-off", or the distance between the exit of the riser and the bottom of whatever surface you are cooking on, is a little touchy.  If there is not enough room for exhaust from your "jet", you will stall the burn process.  If there is too much room, ambient air mixes with the exhaust (your heat source) and you get a less effective cooking experience.  I have found the optimum height to be something between 1 and 2 inches.  Adjusting the height of your surface is one way to control the amount of heat, as is the rate and volume of fuel/air supplied to the system.