The Diagram does not have any strict size, you can assume that the diagram surface is infinite. When you drag a shape outside the diagram, it actually is outside the viewport, but you are able to pan there and see the shape (Read more here.). Or you can also enable the scroll bars of the diagram, which will lead that when you drag a shape outside, the scroll bars will become visible.
You have also one more option - to use our Thumbnail control that visualize the whole diagram including the shapes that are outside the view port.
Please let us know if you have further questions.
the Telerik team