Helpful reminder and great print size/image size break down from Photo Focus:
The easiest solution is to print an image size that matches your pixels. For example, if you want a standard 5×7 print, the image needs to be at least 1500 pixels wide and 2100 pixels tall. The formula is multiplying the inches of your print by 300. So for our 5×7 print, you multiply 5 by 300 to get 1500, and 7 by 300 to get 2100. Instead of doing the math, you can always follow this chart.