Header image height

I’m trying to increase the size (height) of my header image. It is very thin, not like the one shown in the preview. A good height for me would be around 350px. I’ve been digging around for a hours and managed to make it bigger by changing the padding of the menu buttons (I think) but that messes up the drop down links. How do I increase the height of my header image? Thanks.

http://brocksbooks.com/