Header size

It looks like your theme is sizing your header image based on the following CSS:

.header-image {
width: 100%;
height: auto;
}

If you you change the width, it should make the image appear smaller. If you also want it centered (since it displays flush left when it’s smaller) you can add the following to the end of the above code:

.header-image {
width: 100%;
height: auto;
margin: 0 auto;
display: block;
}

I’m not sure this is exactly what you’re looking for, but I hope it helps!