A solid or liquid propellant is typically oxidized, or burned inside a nozzle. The expansion of the fuel into the gaseous state provides thrust against the nozzle surface.
At the very bottom is the nozzle, after that is the fuel. Depending on how big the rocket is and how much fuel it needs determines the size of the chamber
This depends on the fuels used (Liquid Oxygen, Hydrazine, etc) the design contours of the nozzles (if we are talking about liquid fuel rocket engines, and pressure pumps, turbo-chargers etc in the...