There is no "one size fits all" when it comes to distributing your images
for others to display on their own equipment.
Of course you know that the image is just pixels... each pixel of your
image requires a screen dot to display it. A 1024x768 image will perfectly
fill the screen if the system displaying it has that resolution set for
their monitor. If they have, for instance, 1280x1024 set, then your
1024x768 image will take up less than their whole screen, even though it is
still using the same number of pixels. Conversely, if someone has their
display set to 800x600, they would have to scroll to see all of your image.
UNLESS, they use a graphics viewer which automatically resizes the image for
I fear that the truest answer is that it depends on the settings of your
recipients' monitors, as to what size image will display entirely without
Ideally, you would know the display resolution of the other person's
monitor, and use Save For Web to generate a copy of your image with
dimensions appropriate for them, and quality adjusted to make the file size
more comfortable for email.