The rooms are all veery close to the same size unless you booked a suite. They are quite large, however, so you should be very comfortable. The rooms in the atrium can be noisy from the lounge, so I would request a room along one of the two corridors on either side. Room views are all quite pleasant, with nothing spectacular. There are no views of the castle from the SL building. We didn't find our room on the ground floor to be noisy, but we were almost as far as you can get from the lounge. Nice not to have to use the elevator, IMO. There is more privacy on the higher balconies though.
I would request a quiet location away from the lounge and nearest the MK boat dock with a full balcony. (unless you prefer the dormer balconies - but these rooms are slightly smaller). You can also request a nice view, no trees.