You’ve seen them. . . prints from digicams blown up to posters. The pixels are the size of a grapefruit and it’s not pretty.
There are limits to how large you can go. Eventually experience helps you to just “know” these limits, and printer and paper quality play a part in this. But there is a rule of thumb you can use as a starting point.
To calculate (in inches) the largest photo-quality print you can make from a digital file, using its native resolution, meaning no interpolation, divide the vertical and horizontal pixel counts (see your manual) by 240.
If you are making images for commercial publication or other critical applications, or if you want exhibition-quality prints from commercial printers, divide the pixel counts by 300.
Let’s take an image from the Canon PowerShot G6 as an example. It makes a high quality jpeg image at 3072 x 2304 pixels. Using the formula above, that would generate a native print size of 12.8″ x 9.6″ at 240 DPI. Now remember, this assumes you haven’t rezzed up the image in Photoshop.
The next step in the formula is to take this print size and multiply it by 25%. That is the largest amount of image interpolation the picture can stand and still maintain reasonable photo quality – in my opinion.
In the example above, that would change the maximum print size to 16″x 12″.
Note, this is only a GUIDELINE. So don’t make a religion or cult out of my advice. Just use it to get started and do your own experimentation. I think you’ll find that this guideline is pretty accurate.
You can go bigger with some post-processing tricks and add-on software, but the results are far less predictable.