You can control the header height in CSS. The lass name you need is .rsHeader.
However, I suggest that you are going to have to do a lot of other tweaking to get things working the way you want as the background-image and, I suspect, a number of contained controls in the header, are styled on the assumption of a given height. As an example, SS1.jpg shows the Scheduler from this demo page
as is is defined and SS2.jpg shows it with just the height of that item changed.
Remember, by using the developer tools available in most modern browsers you can both discover this information for yourself and play with many of the settings on the fly to see the impact of your changes.
Hope this helps.