Well, it's just a ratio.
So if you're not going to use Landscape mode, the ratio is 8:10. So you could so something like 800 x 1000 pixels.
Or 1600 x 2000, 1200x1500, etc...
Going to landscape should get you an aspect ratio close to many common screen aspect ration (1600x1200, 1280x1024, etc)