Making money online through Adsense and other ad networks

The leaderboard is the 728x90 ad size.   What's nice about this size is that it stretches across the screen.  I've seen some people use this at the very top of their webpage.

A better placement and I suggest you test this, is the place after your header graphic and after your navigation bar.   People is usually ad-blind at the very top of the webpages as this was the usual spot for banner advertising a few years ago.  

The space after the header graphic and navigation bar is closer to the content.  It is also close to your navigation bar where your users mostly click on to move around your site so it is a very prime location for leaderboard.