Website header sizes?

I am just wondering what image sizes are desirable to have for website header?

In my opinion the smaller the better ... I go no more than about 250, and most of the time I'm significantly under that - like 180 or so.

It really depends on what kind of information is in your header and what kind of information is below that on the page. Typically you should be able to at least see that there is other content on the page even at small resolutions. However, that said now that almost every computer has a scroll mouse and users understand that a scroll bar means more information the mythical "fold" carry over from newspaper design is almost irrelevant.

More thoughts...
http://www.boxesandarrows.com/view/blasting-the-myth-of
http://www.viget.com/inspire/evolving-design-standards-the-fold/
http://blog.clicktale.com/?p=19

Thanks guys, all the advices are really helpful for a beginner like me!