There is no definitive answer to this question because the maximum size is determined by your machine (its CPU speed and the available RAM) as well as the structural and visual complexity of the diagram. E.g. rectangular nodes from the "Shape Nodes" palette section do not require as much resources as complex swimlane/table nodes. Moreover, when arranging diagrams with yEd's automatic layout algorithms, the edge-to-node ratio is often more important than the overall element count.
