The size of an image, as you're asking, is not the same as the ppi. A 300ppi(or dpi when used in printers) resolution image is suitable for printing, but the size of the image will determine how large the image is when it's printed. 72 ppi is the standard resolution of images used online, which if printed, will look grainy and pixelated even though the image size may be large. Think of ppi as basically the level of detail, the lower the ppi, the lower the sharpness and overall quality of the image. You cab have a 300ppi image that is 1200px x 3000px or 6000px x 4000px, there is no correlation between them really.