I consider this a bug or at least poor software design.

When I edit the drawing preferences and change the arrow size (under the Dimensions tab), the arrows of my dimensions change as expected, but those of the leaders to not change.

No drawing preference exists to adjust the leaders separately, and scaling a leader does not scale its arrow. I currently must delete my leaders and redraw them completely to change the arrow size. Very inconvenient.

P.S. I'm using QCad 2.0.5.0-Free-1 (Community Edition) and didn't see that version in your list. Does it have another name?