Once the nozzle pressure ratio (Pt/Ps) reaches critical (~1.89), the nozzle throat is choked, and no further velocity (i.e. thrust) is available.
So what you need for designing the nozzle are the gas parameters - Temp, pressure (probably determined by noise limits), gas mass flow... and these will set the nozzle area. The geometry will probably be determined by the position of the gas turbine (aircraft CG considerations) and the distance to the nozzle exit plane. Sometimes there is a smooth ramp of flow area to the exit, and sometimes a constant tubular section back to a convergent section right at the aft end; The former is probably lighter.