As I managed to understand, you have very big images but they not initially shown with their original sizes in the DiagramShapes. If so and if the issue may occur only when the Diagram is zoomed in, please note that zooming in Diagram will only create a ScaleTransform and will not change the Size of the DiagramItems.
However, you can edit the default style of DiagramSettingsPane and update the NumericUpdown controls to increase their Maximum property which is set to 10000.
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