Actually, Photoshop can scale up fairly smoothly. I had to do that for an image that I needed to scale up by about 2.5x when printing it. Even on screen with magnification, the scaled version did not appear pixellated. But yeah, I generally render at final resolution or larger. As a test, I once did a simple scene (a marble sphere on reflecting ground) at 9000 by 9000 pixels, just to see if the P5 render would handle it. (That's the resolution required for a 30x30 print at 300dpi.) It took all night, but it worked.