When we talk about image size in computer graphics, we generally use Pixel measurement.
One pixel is a dot, which only includes a color information, RBG.
The concept of Pixel is friendly to most of programmers. The problem, however, is it is not familiar to most of graphic designers. They are familiar with Inches.
It is because by default Adobe Photoshop, which is the most famous graphic design tool, use inches instead of pixel.
Then how large is 1 inch? The meaning is clear to designer, because it is 1 inch when they print out the picture; it is misunderstand that 1 inch is for the size on their screen. But it is depending on how much delicate the printer is. The more precise printer is, the more detailed picture they can print out.
So this is real question for a programmer. How many pixels is 1 inch? Because programmer should understand when a designer talk in inches world. A programmer need to know DPI, Dot Per Inch. For instance, 300 DPI printer can print out 300 pixels in 1 inch. That means 1 inch is 300 pixel.
Then how could we know how much DPI a designer is talking about? The photoshop show the DPI setting whenever we create a new canvas
Here this screen shot shows 72 DPI, which means 1 inch is 72 pixels; "Resolution" on this screen shot means DPI.Considering game developing, we shouldn't use inch measurement, because inch is for printing out, and most of gamers like to enjoy on screen not on paper.