In the world of computer and printed images, there are three terms that appear time and time again - they are DPI (or dots per inch), resolution and physical printing size. You could say there are four terms if you count file size. All four terms are interrelated, and this node explains that relationship.

Divide the pixel resolution in each dimension by the dpi and you will get the default physical print size. eg. a 1856x1392 pixel image at 72 dpi will yield an image that prints at 25.8 x 19.3 inches.

Lots of graphics software will allow you to either ignore the dpi setting or change the dpi setting to yield a default printing size that is what you want. Using the image from before and changing its dpi setting to 300 dpi would result in a default print size of 6.2x4.6 inches.

I keep saying "default print size", because many applications have a page setup option that will allow you to select any size you want for the output.

The dpi setting has *no* effect on images on the web.

File size depends only on the pixel resolution, and not on the dpi setting. Once again using the previous example, the uncompressed image file will have 2.6 million (1856x1392) sets of three numbers defining the red, green, and blue value for each of of the 2.6 million pixels of your image. The higher the pixel resolution, the bigger the file. One can use compression schemes that remove redundancy or perhaps less obvious image details to decrease file size. JPG is such a scheme that can create image sizes that are ten times smaller without a blatant decrease in quality.