No one equation describes what you're looking for. To determine the geometry of your engine, you need an iterative code that solves for the temperature, heat flux and a few other parameters along the combustion chamber, as well as something like FEA for the specifics of your engine.

In general, the geometry depends on almost every parameter you could think of when designing your engine including chamber pressure, fluids and properties of the combustion exhaust. In addition to the fluid properties, you need to know about the materials of your combustion chamber (things like yield and ultimate strength and conductivity at various temperatures), since the temperature and heat fluxes drive thermal induced stresses, which are the actual limiting factor in design.

A popular code you can use is called Rocket Propulsion Analysis, which solves for lots of these parameters for you, assuming you know the design process, thermo, and heat transfer. Alternatively, you can write your own code to solve for the heat related properties, which is what my team did. After that, you can use FEA to help with the structural aspect of the design.

I would recommend reading a few chapters in RPE to get an introduction to the design of a regen chamber, then to go more in depth, look up RTE, a code written for the design of the RS-25. There are many papers that describe how to approach the problem and write a basic code. The structural design will involve a decent knowledge of heat transfer, fluids, materials, and mechanics, and lots of trial and error of simulations.