How to Change Header Height?

Working with the Beaver Child theme. How can I make the header height taller than padding will allow? I want to use a larger picture and I want it spread out across the whole header and not cut off. Also what size image would I use? Right now when I am viewing in wordpress the image is not cut off. But when I publish it 1/3 is cut off horizontally.
I hope that makes sense. http://www.jeanetteblix.com 2
thnx!