The landing page quality guidelines for AdWords MAY give some insight into how this will work. They state that, in a 1024 x 768 maximised window:
1. At least 30% of the above-the-fold area must be actual content. Content excludes headers, navigation, search boxes, etc.
2. Ad area above-the-fold must be less than or equal to actual content.
See the details in the 5th post on the following page: