Hi Jennie,
Thanks for joining
It is a little tricky because of the way that we have tried to economise on images used. So the way to achieve what you want to do is:
1. Change the height of #top_wrap to suit the height you want the top area to be.
2. Edit the top.jpg image to the new height so that you add space to the top of the file and that the thin line that separates the nav area is in the bottom third of the image.
So if you want the top area to be 50px taller than it is now then you need to add 50px to the top part of the top.jpg image.
Let me know how you go.
Thanks Anthony