I am quite new to dia, so I do not have any experience with the tool.
I would need to have shapes in the form of block-arrows as in the various category, with one major difference: The angles of the heads/tails shall be fixed.
Just for explanation: if the block-arrows are resized the angle changes is the resizing is not proportional. So extending the length the arrow head becomes smaller and longer and the angle flatter.
I have seen a similar shape with the required attributes as the “KAOS Agent” where the shape can be resized but the left and right side angles are constant.