If I understand correctly, line style is partially defined in this file:
https://github.com/FreeCAD/FreeCAD/blob ... eGroup.csv
in part :
;Format of this file: *GroupName,thin,graphic,thick,extra
But I do not know the details, and what exactly can be changed in this file.
It seems that you can only change the thickness there, but you can't change the dot-dash distance.
You can select the desired style group in the TechDarw settings, depending on the size of the drawing sheet.