I just got the same notice. My first response was "D-23 is in a world of hurt!"
Looking at floor plans, the three major sections of the Fantasia Ballroom have a Theater Style seating capacity of 1,090 each. This does not take into account any space for a stage and screens. Att he three main sections and you have capacity for over 3,000 people. If you consider the entire ballroom, with all ten of the maller breakouts, the capacity is over 4,000.
Now they did have about 1,300 attendees at the Destination D in DLR last September. I imagine their target was 2,500 for WDW, as this would allow for all the seating, plus the areas for the stage, A/V control, etc. I am sure they have not reached this and are now screaming.