are you using MS Office Picture Manager? If so...
Look for Export...possibly along the right side of the screen. Then you just set the custom settings like you want them. As mentioned previously, definitely reduce the size for something viewable on a monitor (usually no bigger than 800x600 for web, forums, emails, etc (forums...really a bit smaller)
Then you can set your JPEG quality. Setting it lower will compress the image (filesize) smaller, but not necessarily a better quality. Also, setting the JPEG quality lower will NOT make the viewable size any smaller. In MS Picture Manager, I have found that 80 or above is usually "decent", while at 90 the blemishes are barely noticeable. Of course, this depends on your photo...just give it a try, and make sure not to overwrite your originals!