When sending a diagram to the printer in Toad's ER Diagrammer, nothing gets sent to the printer and nothing gets printed. This only happens to some of the diagrams.
Toad has limited functionality with ER diagrams. It will not print an ERD if it is too big. There is no set size limit to quote since this depends on table number and size, and scale setting.
Try reducing the number of tables or table size, and reduce the scale of the diagram to one that is printable.
Try saving as a bitmap and then printing the bitmap. There is an icon on the ERD toolbar to save as bitmap. But again it is limited, so if the ERD is too big it won't save, giving a "Not enough storage memory" or spool size error.
If neither of these suggestions are suitable for your diagramming needs please try Quest's ERD application like Toad Data Modeler.