Digital Image Basics for non-Photographers • Blaise Tobia
If an image will be viewed on screen via a browser or image display application,
then the only necessary size specification is its pixel dimensions.
For example: if it's desired that an image should be no bigger than 14x14 inches
as viewed on a standard monitor screen, then the size specification should be:
Images should be no more than 1008 pixels in either dimension.
This should, of course, be augmented by specification of file type, quality, etc.
Note that there is no difference in the size of image needed for projection.
One projected inch is the same as one screen inch in terms of the necessary resolution
(even if the projected inch can be measured as a foot or more at the projection screen).
Graph of Display Resolutions and Designations