Make the size of the grid larger than the room. Use either a Space object or a closed LWPOLYLINE to serve as the boundary for the grid.
Try to keep the grid size at or close to the minimum required. While the grid will only show inside the clipping boundary, the full grid is maintained in the file/in memory. You can make a file get really huge in a hurry if you make all of your ceiling grids 100'x100', rather than working out the minimum each space needs.
David Koch
AutoCAD Architecture and Revit User
Blog | LinkedIn