Use the largest pic you are willing to render and enlarge it with PS or PSP. Normally when you let it print then the pixels are just enlarged. If you use a smartsize (or whatever works best for you) then the room between pixels will be interpolated and most of the time it looks better if printed then. Finally add some filters to it that smooth, sharpen, clarify etc... the newly sized pic and you could get away with a smaller render and still have a good quality print.