The resolution measure of computers is dpi which is short for Dots per Inch. A fair laser printer prints in 300dpi, and a good printer uses 600dpi.
The dpi value for computer screens is a tricky business. There are screens of different physical size as well as computers with different screen resolutions. Screen resolution is usually given in width and height of the screen (in pixels). Usually, 72dpi is a good assumption for your screen resolution to start with.
The choices for picture size depend on your scanning software. If you are lucky, you will see the size of the image given in pixels, but there are scanner modules that just display the height and width of the scanned frame in inches together with the selected resolution. In this case, it is up to you to use a calculator to multiply the inches with the dpi value to gain the pixel number.
If your image is too small after you scanned it, it is preferable to rescan it at a higher resolution (= more dots per inch) rather than enlarging it within your graphics software. The reason is quality: if you blow up an image, the computer has to guess what colors it must give to the new pixels inbetween two of the scanned pixels.
Images that are too large can be downsized with good graphics packages without loss of quality.
In general, you should know what resolution you will need the image in before you scan it. Remember that most computers on the World Wide Web support screen resolutions between 640x480 and 1024x768.