The backend uses RadWindow which is re sized depending on the browser and its resolution. What is the browser you use and its current resolution? If you use another browser and resolution 1680 x 1050 do you have the same issues? Does using the built-in template( master page) make a difference? It is possible that you have something in your template that causes this white space.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items