Just as physical images are measured in inches or centimeters, digital photos are measured in pixels. This unit of measurement is most commonly used in graphic design, web design, and digital photography, and pixel size refers to the digital measurement of pixels in the image to ensure that an image or element within an image will display the same size regardless of the screen resolution that it’s being viewed on.
What is the size of a pixel? You’ll have to use a pixel calculator to determine that, which will allow you to identify more than just the size of each pixel in the image, including the pixel density and pixel count.
Is pixel size important? Absolutely. Pixels capture light and turn it into data. So, the bigger a pixel is, the more data it is essentially collecting. If all other factors are the same, a bigger pixel size means that the image or display will be higher-quality.
In terms of photography, for example, larger sensor sizes allow for larger pixel sizes. This, in turn, leads to higher-quality photos. The pixel size for a Nikon D700 is 8.45µm whereas the pixel size for an Apple iPhone 5 is only 1.4µm.