A resolution of 300 to 600 dpi should be selected only if you wish to print the image. For the web, a scan of 72 dpi is sufficient. Use the color depth you need.
File size increases with the increasing number of colors.
Use the scanner software to preview the scan, then select the area to scan. While this is not particularly useful at lower scan resolutions, at higher resolutions, it will reduce scan time if you only want to scan part of an image. Newer scanners also have a one-button scan feature located on the scanner that send a scanned image directly to the computer without using software. If you need to edit the image after scanning, save it in photoshop or TIFF format. JPEGs for photographs, and GIFs for images containing solid colors.