When the question how big is a photo 4x6, it is usually talking in terms of a print size of 4 x 6. Digitally, pixel size is dependent on resolution. A 4x6 size photograph has 1200 x 1800 pixels at 300 DPI (print quality). It is approximately 288 x 432 pixels at 72 DPI (screen use). Visuals Clipping explains that learning the size of a photo 4x6 should be used to make sure that you have the right size to print and post online.