The Disk Capacity Meter, located at the right end of the bottom bar, shows an estimate of the DVD space used by your project. The exact capacity is not known until the burn process has started. Regardless of the estimate, you can choose to attempt to burn your project to DVD.
When the meter appears white the estimate is based on the capacity of a standard single layer DVD (4.7GB).
When the meter appears blue the estimate is based on the capacity of a dual layer DVD (8.5GB).
When the meter appears red the estimate is that your project exceeds the capacity of a dual layer DVD (8.5GB).