Comment by danbruc

It is neither a unit of length nor area, it is just a count, a pixel - ignoring the CSS pixel - has no inherent length or area. To get from the number of pixels to a length or area, you need the pixel density. 1920 pixel divided by 300 pixel per inch gives you the length of 6.4 inch and it all is dimensionally consistent. The same for 18 mega pixel, with a density of 300 times 300 pixel per square inch you get an image area of 200 square inch. Here pixel per inch times pixel per inch becomes pixel per square inch, not square pixel per square inch.