Say if you really want to make the most of the screen space for your header, what maximum width would you go for? To the point where your not leaving much gap outside of it.
Well the thing with that is people use different size resolutions. 800x600 is pretty much dead, but the smallest common one is now 1024x768 (mobile devices are still smaller obviously.) What you can do is create a big version and then also a mobile version, I am pretty sure there is a plugin which does that for you. If you really want the page to take up most of the screen on all resolutions the best way to go is with fluid width (a percent.) It depends on how your theme is coded though. There are also advantages/disadvantages to both ways.
Regarding fluid width im guessing thats if the header will purely be styled with CSS ( im currently learning ) so if using a header image then fluid width doesn't come into it?
Well again it depends . You can still have a header image if you use fluid width either if your header is not fluid or if your image is tileable or if you just simply center it or align it to one side. Even if your width is not fluid your theme will still be using css. To see and edit your css you can go to \wp-content\themes\'theme name'\ and usually in that file you will see a style.css. Some themes use more than one css file so you may have a css folder or something with a few css files. Most of the ones do it the simple way though with just one.
Thanks myp, im just trying to get this all to sink in, once i start understanding what im doing more il be more comfortable with it. Reading at wordpress codex im reading "If you are using a header image that can be repeated, and you are using elastic widths, you can set the styles within the header to repeat to fill the space" When they talk of elastic widths are they meaning fluid?
Elastic widths are a combination of both fluid and fixed. Here is a quick example: http://www.456bereastreet.com/archive/200504/fixed_or_fluid_width_elastic/
Thanks very much, il have a good read of that, thanks for the link, it looks very useful. Im learning something good everyday.