Apologies for the delay in the answer.
It's not possible in EnSight to change the size of bar2 elements based on a scalar field. Indeed the bar2 elements size is given by the position of the beginning and end node of the element itself.
The only thing you can do, is apply a shrink factor to separate each bar2 element from its neighbors. This may help you in the visualization. To do this: select the part in the part list, right-click -> Edit. In the widget that opens, toggle on the "Advanced" and, under "Node, element and line" change "Shrink factory". By default its value is 0 (no shrink). You can increase it up to 1.0 ( = 100 % shrink).
Hope this helps.