The largest/longest/widest diminision would be 4096, the smallest/shortest/thinest diminision should be in proportion to the largest. I suggest you make sure the proportions of the work window are correct for the 16 x 20 size. It will prevent surprises like having to trim art off the sides of your work, or having to to enlarge the image more than intended to fill gaps. Be carefull of background resolations (they tend to blurr). If you put backgrounds in postwork carefull of low res (again you get blurring). If you are going to enlarge images a lot, you might consider a Fractal program. Note you can set up photoshop to let you work with a CYMK colors, in RGB mode (so you can use filters etc), IF CYMK is required for printing.