There probably can never be a perfect image size for people to view on their monitors, since monitor size and resolution vary greatly from viewer to viewer. But there are some guidelines we can follow so the images will look reasonably good in most of the cases.
Resolution is a more relevant issue than monitor size.
A good compromise is to assume that the viewers have monitors with 768X1024 resolution. You can view it on an 600X800 monitor without scrolling, and it will not be too small on a 900X1152 monitor. Somewhat larger than 400X600 should be OK in most cases. For portrait, you can have it up to 550 pixels tall. Don't be confused by what you see when the image is shown by an image editor.