The resolution of a large image is highly subjective. A "large" image could refer to the physical size of a printed image, the virtual size of an online image, or the storage size of a saved image.
A good ballpark figure for printed images is 300 pixels per inch to prevent distortion. At this level, a 4 x 6 photo should be printed at a resolution of 1200 x 1800, and an 11 x 14 photo should be printed at 3300 x 4200. An image posted online, however, needs only to be tailored depending on the size of the screen from which it is intended to be viewed. A good rule of thumb for online images is 640 x 480 pixels. The storage size of an image is dependent on its scan resolution, pixel size and the colors used. A 4 x 6 24-bit color photo scanned at 300 pixels per inch with a resolution of 1200 x 1800 would have a file size of about 6.17 megabytes.