We get this question all the time, especially from users of Blow Up. Here is a guide to choosing print size based on your original image size.
Most people consider 300dpi to be very high-quality printing. Dpi is the dots per inch. You may also hear people call it ppi, which is pixels per inch. Divide pixel dimensions by dpi to find out how large you can print an image. So, a 6000 x 4200 pixel image can be printed at 20 x 14 inches. Depressed by this calculation? Don't worry, we can relax it in a few ways.
Our Blow Up software can enlarge a photo by a factor of 2 on each side without any decrease in quality. Many people are happy with factors of 3 or 4. Depending on the image you might be able to go much larger.
You can see the detail of 300dpi glossy paper when you view it close, like within 2 feet. If the viewer is farther away, then you can use lower dpi. Basically, you can divide your dpi by a factor that is the viewing distance divided by 2 feet. If your viewers can’t get closer than 10 feet, then you could print at 60dpi (divided 300dpi by 5 because viewers are five times farther away than 2 feet). That could give you a big boost, but you need to know the viewing situation. Billboards are often printed at 10dpi, because you can't see the huge blocky pixels from the road.
Glossy paper shows off detail, and it is usually best to use 300dpi on it. However, matte paper, fine art paper, and canvas allow ink to spread a little. That slight softening blurs 300dpi detail, so you can get away with lower resolution. In our experience, we couldn’t see much difference between 300dpi and 180dpi on those materials. This gives you a print 1.7 times larger in each dimension. Whatever dpi you were going to use on glossy paper, divide it by 1.7.
However, if you are already lowering dpi because of viewing distance, you probably shouldn't lower it much more because of paper type. That is because the paper blurring can only be seen from a few feet away.
Blow Up does some extra sharpening to compensate for this ink spread: more for matte, less for luster, and least for glossy. This is usually referred to as output sharpening.
Let's look at a typical situation. Suppose you have a photo from a Canon Rebel XTi that is 3888 x 2592 pixels.
Enlarge it with Blow Up by a factor of 2. Now we are at 7776 x 5184 pixels.
Suppose the print will be on a wall where viewers can't get closer than 4 feet. We can lower the dpi by a factor of 2 (4 feet divided by our standard 2 feet viewing distance). Now we are at 150dpi.
Even if we are printing on canvas, let's not lower the dpi more (or not much more) since we already lowered it for viewing distance.
Divide the 7776 x 5184 pixels by 150dpi to get a print of about 52 x 35 inches. Depending on your image content, you might be able to enlarge more with Blow Up or lower the dpi more. The best thing to do is to try a sample area, print it, stick it on the wall, and stand back.
If you'd like to try Blow Up, just download it and use it in trial mode. There is no substitute for an actual test on your own photo.