Is python accepted:
import pcbnew
board = pcbnew.GetBoard()
for drw in board.GetDrawings():
if drw.IsSelected():
drw.SetWidth(int(0.1 * 1e6))
Above snippet sets line width for all selected drawing elements (lines etc.) to 0.1 mm. Note that 1e6 multiplier is required because internal units of kicad is nanometers.