^ IOW, an image saved at 2" across at 72dpi is 144 pixels across. The monitor resolution of the person viewing dictates the size of the image on their screen. You could save the image at 1" across and 144 dpi or 14.4 inches and 10 dpi andthe image would look the same on a monitor.
As a result, I ignore any dimensions but pixels. 640 pixels on the long side is a decent size for gallery viewing, but can be a bit small for some images with lots of detail.
If I understand "le vrai rdu" properly (not sure I do), a real concern is the loss of image quality as it is resized. I find that a bit of trial and error is necessary, but in general, resizing by 1/2 or 1/4 in steps until the desired size is approximated works best. I experiment with "sharper" or "smoother" bicubic settings, but the difference is often subtle. Any time you resize a scan you lose detail, and then when you convert to jpg you lose color and tonal accuracy, so it's an art itself rather than something with a single perfect anser for each image. And one must accept some loss somewhere, or never share images.
