Ok first you have to set a ratio in the document setup that matches the dimensions you want in your render to disk. (BTW you can render 1600 x 2000 without using render to disk, but it may not fit your screen). 16 x 20 is a 4 to 5 ratio. Set this in the Document setup, to be able to see all the scene you can set it to 400 x 500 (which is still 4:5 ratio). Now when you go to render to disk you should get the right dimensions. Was that clear or did I confuse you?