I built my site on my desktop 600x800 but it looks small on my dell laptop with widescreen 1280 x 800. What can I do to make it look good on all computers if anything. Any suggestions would be greatly appreciated.

I don't think it will cause much trouble if you make the site for 1024 x 768, then to me it seems that most people have larger monitors nowadays. With widescreen monitors it might still look small.

You could center the contents of your site and have the same (or at least matching) background colour both inside and outside the contents area. I think that would be the best solution.

There is another way, which I don't find particularly good. That is to set the page width in % instead of px. Then the contents will adjust itself to a set percentage of any screen. But your text and pictures will wrap itself to long or short lines, and the contents may not be displayed correctly (= in the same way) on monitors of different sizes.