Look at mine compared to yours and you see the difference in height size.
If you build fixed size areas you must properly identify height and width (sometimes also the z-index)
Yours was way to small to get the header straight, IE just ignores your settings and resizes the <div> header so that the font fits.
Instead you could use the min-height which makes it sizeable but with a minimal height.
As an example look at the header of following page of mine in all your different browsers
http://dragonflycms.com/