To change the height of the header in this skin you will need to adjust the height of a few images and change the height of the #wrapheader selector. The images you need to adjust are bg.gif and headerbg.gif. When modifying their height, you should add the desired amount of pixels to the top of the images. After the images are the size you want, you need to set the height of #wrapheader to the same as the height of the headerbg.gif image.
For example, if you want to increase the height of the header by 15px, you should add 15px to the top of each image and then set the height of #wrapheader to 100px (because it is currently 85px).