Read out the plot width(s) by setting the active plot using the property "active plot" and read out the width of that plot using the property plot Line.Width...if you find that any of the plots have changed width - write that widt to all plots (set active plot -> write width...next plot..).
If you want the user to control the width just one place (not from the plot legend e.g.) then create a width menu and a for-loop where you set the active plot to the loop index and write the new width...with the number of iterations set to the number of plots...
Mads