The allure of the big school is enticing. My experience (large 4A head) is that you should probably work 18 hour days on average to keep up with the demand. The political environment becomes much more important than teaching. Make sure you know how staffing works with regards to how you are allowed to delegate duties and how much micromanagement you will get from your bosses. Best of luck but remember that the grass is not always greener on the other side.