X3. Our dining room is 12'8" x 14'6". It was originally 13'8" x 15'6", but we took 1' out of it both directions, and I wish that we had not done that. It still works well, but a little extra room would be nice since we host most holiday family dinners at our house. Another thing I noticed is your rear porch is going to feel closed in with walls of the house on three sides. I would definitely change that. You will want the rear porch to be open on all three sides, or at least two sides. I would also make the great room larger. You are going to lose 3'-4' of your great room as basically a hallway to the master bedroom and office. One thing that helped me tremendously was to get 1/4"=1' printed out plans from our architect, and then cut out pieces of construction paper to scale that represented each peice of furniture. Place the pieces of construction paper furniture on the plans and look at how much room you have around them to walk, access doors etc.