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

There are not really any limits because it really depends on the design you are working with. On average, I range from 200px to 300px tall which includes the navigation. The width is up to you, it is a good idea to do max/min setups with css but then you are sometimes limited to the type of graphics you use.

Typically, I try to keep my headers no more than 800 pix wide. I agree with the previous reply of 200 - 300 pix in height though. You really don't want them to be too big.

It all depends. Having to scroll 600px down on a Iphone just to see the content is quite annoying, so personally I would stick to a small 30-75px header (with or without menu/search bar).

It obviously depends on the context. As mentioned by Ulrik, if you are catering for mobile users, then you'd want a small banner/header. Generally, for PCs, I create headers that are between 200 and 300px as anything much larger than this tends to force the visitors to scroll a lot on the page, particularly with screen resolutions of 1024x768 or 800x600.

Width-wise, I usually create banners smaller than 900px. 800px is good, although it can look a bit small on a larger screen. As mentioned, using CSS can overcome this - either by setting a max and min-width; a banner that grows when the window is resized, or a combination of them.

Dan