What you can do is that if you know exactly what your max size printing is, set up your canvas at that -- i.e.: 8.5"x11" at 300dpi. Pepper's point is quite true as well. You can always downsize a pic, which you WILL lose finer details. BUT, it will look HELLA better than if you try to enlarge pics. Enlarging pics is really bad and if you can, avoid doing so as much as possible.
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Also, with larger sizes, that you shrink down, you can get away with being slightly messier since as you shrink it down, minor mistakes can be covered up.