You will notice as you zoom in & out on the screen the dots/dashes remain the same visual size. The same happens on the plot, dots & dashes are the same length on paper for full sheet and plotted zoomed-in area. I think
this is a function of the graphics toolkit (QT-4(?)). Somewhere there is/was? a control for the relative lengths of line and space in Medusa at the project level, but I suspect it's been superseded, and in any case Medusa Personal doesn't allow us to change that sort of thing.
It might be worth digging into the QT-4 source documentation, but I don't think it's going to help much. http://doc.qt.io/qt-4.8/qpen.html