Scan the photo at the correct size Try to scan the photo at a size and resolution greater than you can imagine using it. This will take up more space, but will ultimately make the photo better able to be used in a wider variety of projects. More on this below. The total number of pixels in an image determines teh image's file size. This not the complete story, though.
Image size is a function of the number of pixels, but also of the amount of data at each pixel.
I will use ppi and dpi interchangably. In general, a higher resolution translates to better, more detailed print quality. Indeed, if you are going to use an image exclusively for display on computers, the display technology can not take advantage of the added information. 200-250 ppi For most professional offset presses and direct printing of items such as color magazines or brochures. In general, you should try to downsample your images rather than resample up.