If you go to the RCCL website and look at the ship you are interested in, you will find staterioom floor plans and square footages for the cabins and the balconies. There are also 360 degree tours of the staterooms.
I agree to use the website because there is no set answer with the different classes of ships in the "RCI" fleet.
And about a "JS" cabin....Now that I have love the longer itineraries, I go for the larger rooms (budget allowing of course). In May, seven days on "Serenade" we have an "E1" but in September, fourteen days on "Radiance" we have a "JS." Busier the itinerary, smaller the room. Hope that helps, Robbie