Re: How do I know what size logo to use for my header?
If you want the page to be fluid, but still look good in most browsers, I would make the logo no more than 750 pixels wide and make the background of the header the same as the background of the logo. The header background can then be repeated behind the logo for a seamless look.
Please do not PM for support issues: a private solution doesn't benefit the community.
Be careful with unsolicited advice via email or PM - Make sure the person you are talking to is a reliable source.