I’m not sure about the sizing limitations for it. It just removes the default margins and moves the header image up and to the left. I use it for mine, but like jandupamu said before, I have a weird header size. I believe mine is around 850 x 285. They talk about some other possible fixes in this thread: http://www.onedesigns.com/support/topic/customize-your-header-tutorial-and-question
But again, they were having mixed results.