What I do is build print templates. I make a solid white document, the size I want the image to print. Then I increase the DPI to 300. This of course increases the file size. But when you increase the DPI you also get a box to enter the size you want the image to print at. I have templates of all the sizes I use, and just build my images on those.

Talent is God's gift to you. Using it is your gift to God.