Paul, when resizing for web viewing, forget the inches altogether and use
the image dimension in pixels; you want an image that's in the vicinity of
800 pixels by 600 pixels. Monitors speak in pixels; they don't know about
inches; printers know inches and they work best when the resolution is 200
ppi or more.

If you're wanting to size for the web using Image>Resize>Image Size, you're
going to need to put the check in the Resample box, then adjust the Image
Dimensions in the top section. By doing so, you'll be downsampling (getting
rid of excess pixels) so make sure you've saved your original image with all
its pixels intact before you take the step - if you ever want to print the
image with acceptable resolution. Personally, I use the Save for Web
feature to create web/e-mail size copies - pretty straightforward and gives
you a chance to save the image under a different name - no sweat!