Hi Marty, as Ralph mentioned it can be tricky picking a background size as these days there are multiple considerations given display sizes and resolution. What you can do is employ some visual trickery in order to give the impression of depth in the background without needing to try and fill it completely. This is often done as Ralph mentioned by fading out the edges of your image to a solid background colour. You often see it with a glow-type effect centred at the top of a website. It picks out the header area and gives some weight and visual punch without needing to take up the whole background. If your website is likely to have long pages that scroll vertically and you want to have the effect present, you can do a similar thing but apply it to the edge of your website's frame/body content.
Photographic backgrounds can be tricky and I often see them used at resolutions that are too small on my 27" iMac. I know what they're supposed to look like, and would look like at different resolutions but often I see 80-90% of the image and then a nasty repeat both horizontally and vertically.
The other advantage to providing part of an image with a fade is bandwidth. You get the benefit of an image without needing to download a large background image file or having to provide multiple background files for different screens.