There’s only one situation in which inches are a useful measure of digital images: when you print them. We measure the size of the paper it is printed on, such as a 8.5 x 11 inch sheet of letter-size paper, or a 4 x 6 inch photograph.
While the quality of a printed image can be measured by counting how many dots-per-inch (DPI) were used to print it. Dots-per-inch means what is says: 300 DPI means each square inch of the image is made by printing 300 tiny dots across by 300 tiny dots high.
If you want to print a poster 11 inches wide and 17 inches tall, and your printer is set to print 300 dots per inch, than ideally your digital image should be 3300 pixels wide by 5100 high. If your printer outputs a mere 100 pixels per inch, than the image need only be 1100 x 1700.
But if you’re not printing, don’t worry about, don’t even think about inches. For you, there are no inches.
