Its not so much the proportion that matters but the resolution of it. I have a makoto picture that 1200x800 at a resolution of 300 that looks amazing when expanded a bit. But I other pictures that don't look so well when they have a resolution of 75. So try and find something when you open of photoshop and look at the dimension that has a higher resolution, it will expand and look better. But overall its just how much grainyness/ slight blurring you want. IF you want a crisp picture find something big, if you don't mind some blurred colors blow up an image. Its your stick go nuts.
I'm a poor photoshopper so hopefully other people can give better advice.