You can set the size of a diagram on the Editor.

Click "Properties" in the toolbar and set the size by where it says “Sheet Size.”

When you’re finished updating the size, click “Update.”

The maximum value for each width and height is 8,000 pixels.

Please note: On the Flash version of the Editor, the combined width and height of the diagram cannot exceed 16,000,000 pixels. (For example, 4,000 pixels x 4,000 pixels is ok, 2,000 pixels x 8,000 pixels is also ok, but 8,000 x 8,000 is not possible.)