I believe you'll find an image size of 320 x 240 pixels generally suffices for web display on forums, though with a highly detailed image, 640 x 480 may be preferable. As mentioned above, IrfanView makes specific resizing an image to a given pixel x pixel size, or resizing the image to a given percentage of original size, up or down (though for various reasons having nothing to do with IrfanView, sizing an image up typically yields less-than-satisfactory results) a very simple "just click" thing.
As rule-of-thumb, considering bandwidth and page-loading time with a mind to those saddled with dial-up connections, a maximum of 800 x 600 is a good idea. If you really want viewers to experience the full impact of a larger image, allowance may be made for the bandwidth-challenged through employment of a free image hosting site, such as
ImageShack, a feature of which allows the posting of a clickable thumbnail permitting those so inclined to access the full-sized image while not requiring all to load the entire file upon viewing the page incorporating the clickable thumbnail.
Here's an example:
Clicking the thumbnail will redirect you to a page displaying the full-sized image. Depending on your browser, you may have to click on the image once more after arriving at the page to display it full size, and unless your monitor is truly huge, you won't be able to see the entire image without scrolling.