When saving to file, how can I set the screen resolution to 800x600 for the image contest? Is it that option in Advanced where it says custom resolution? My image takes awhile to render, so I just want to know before I go.

Yes. Actually, if you open up the disclosure triangle for the resolution, you can type in whatever values you want and it'll automatically switch to "Custom".

If you want piece of mind then you could render out really small first as a test.

Ya, lol. I am rendering small for tests, but I want to see what resolution works for this image.