System heap size is not an issue as far as DAs are concerned under System 6.
When MultiFinder is not active, DAs are loaded into the application heap. With
MultiFinder active, DAs are open and closed via the DA Handler. The DA Handler
loads the DAs into the system heap, whose size is automatically increased by
MultiFinder, as necessary. When a DA is closed, MultiFinder reduces the size
of the system heap.