As the size of the pictures in your project can impose a limit to high-quality digital projection, you should rather scan your pictures at a larger size. Digital photographs should be used at their original size. This will give you the every chance to create videos or presentations with smaller pictures at a later time.
The appropriate minimum size depends on your applications. Here are a few examples:
Videos up to DVD standard... 800 ... 1500 pixels picture width
Digital projection at highest HDAV quality... 2000 ... 4000 pixels picture width
The picture width you choose depends on the maximum requirement for future applications. This, however, also requires high-quality image capturing devices (video cameras, scanners, digital cameras, etc.). If you are planning to create extensive camera movements, the pictures need to be scanned at a larger size in accordance with the travelling path of the camera or the zoom factor. See also Picture size for camera movements.
Tip: If restricted memory capacity is no major issue, we recommend using TIF format for the pictures, Advantages of the TIF format.
See also
Pixels, dpi, ppi - Picture size and resolution