Resolution: measure of output image quality generally expressed as PPI pixels per inch (screens) or DPI dots per inch (printers). Higher resolution = greater capture of detail. Screens typically display at 72 to 100 PPI. Photo-quality ink jet printers typically print at 300. (To add to the general confusion, printers sometimes refer to DPI at 1440 or 2880, but these are ink dots, not pixel equivalents.)
Size: depends on the resolution of the device the file is being displayed upon. A file with pixel dimensions of 800 x 1200 pixels would display at 8 x 12 inches on a screen with a resolution of 100 PPI, but it would print at 2.66 x 4 inches on a printer with a resolution of 300 DPI.