The Great Movie Ride only holds 225 people. The Land Pavilion is not listed as an event venue on Disney Meetings' website.
If the event is really for 1,500+ people, my guess is either the Wonders of Life Pavilion or the World Showplace Pavilion behind Canada, since both are regularly used special-event venues that hold a huge number of people.
You can narrow down the choices by checking out all the available theme park event venues on Disney Meetings' interactive map, here:
http://adisneymeetings.disney.go.com/media/dwm/en_US/media/tools/ipm/IPM.html
The site also has floorplans and capacities for the convention centers, but I'm thinking the DIS wouldn't settle for anything as staid as a ballroom.