@Rob Sir,
Here is my poor solution (As my residuals didn't even converge to 1e-3 and mass imbalance was in order of 2) I obtained using 2D mesh just to give an idea why I am making such a large domain due to presence of shocks or expansion waves.
(2D Domain size was 10000 mm * 6000 mm)
Above is the image showing, How I tried to use body of influence using elliptical shape.
Nozzle domain extent (I didn't use scaling, I just change the view length unit in mm).
Domain Extent for original domain using body of influence sizing of 50mm in meshing.
Domain Extent for shorter domain using default settings in meshing.
I tried running my simulation for all this above coarser jet domain but residuals were not even converging in order of 3 and mass imbalance was closer to 3 or 4. That's why I think i should refine the jet region more but unable to do so.