Decreasing the height of the header & adding image as the whole width of header

I recently added this theme as I like the easy style of it but I came across two things I can’t figure out how to do.
The first is the option to narrow down the header as it’s too tall as it is, it takes a quarter of my screen – how can I do that?
And second question, also about the header. How can I add a header image which is going the whole length of the header, not only 300 pixels wide. Or if this is not possible, is there a way to add other images to the right of the image? Right now it’s a big empty place that just begs to be filled with something.
Thank you!