When printing a digital image the image’s dimensions and resolution are used to determine the maximum print size the image can support. If an image has a large pixel dimension but a low resolution it may print well as a 3.5x5 but would probably not be suitable for a 16x20 print. The best way to reference the maximum possible print size is to compare the image’s dimensions and resolution to this pixel chart which is available as a downloadable PDF at the bottom of this page.
If the image is not fitting into the chosen print size then the aspect ratio of the image may not match the print size. Aspect ratio describes the shape of a rectangle in mathematical terms. The most common aspect ratio for digital images is 2:3. Ratio is expressed in the form x:x such as 3:4 or 2:3 and is spoken as a ratio of three to four or a ratio of two to three. The aspect ratio assumes the image has been divided into equal sized squares. If the ratio is 2:3 that means the image would be two squares high by three squares long.
If you are not sure of the aspect ratio of your image you can divide the long side by the short side to get the ratio expressed as a number; for an image with a ratio of 2:3 that number would be 1.5. You can then use this chart to reference the print sizes available that match your image’s aspect ratio. The chart is also available as a PDF at the bottom of the page.