How Print Size Affects Image Quality
When you set the print size, you need to consider the quality you’ll get at that size. The larger you print an image, the less defined it becomes. So, the print size plays a big part in the quality of the image you get (as illustrated in this image):
Take resolution — pixels per inch —into account.
When you enlarge an image, one of two things happens: The resolution goes down and the pixel size increases, or the software upsamples the image. Similarly, when you reduce print size, you can retain the current pixel count, which makes the resolution go up and the pixel size shrink; or you can retain the current resolution, in which case, the image-editing software downsamples the image.
Figure out the maximum size at which you can print your image at a desired resolution.
Divide the horizontal pixel count (the number of pixels across) by the desired resolution. The result gives you the maximum print width (in inches). To determine the maximum print height, divide the vertical pixel count by the desired resolution.
Decide whether size or resolution is most important.
Notice that the small image (left) has a much higher image quality than the large image (right).
If you don’t have enough pixels to get both the print size and resolution you want, you have to decide between the two which is more important.
Some print utilities offer guidance about how large you can print your picture without losing quality; pay attention to those on-screen messages, and you’ll be fine.