15.64.6.2  SIZE   - SIZE=n in inches sets the half width of the bar at the
end of error bars.  If n is negative n is the fraction of the symbol  size
on  the  data point.  If unspecified it is automatically set to 1/2 of the
symbol size.

In  3-D  plots  the  bar always is perpendicular to the error and to the Z
axis.  This should make the directions fairly easy to see.
(Default:0.1)