To Scale an Actor Interactively

Scale should be turned on for X,Y and Z axes for the actor in Modify Actor tool setting.

Select the Manipulate Actor tool ( Visualization > Animate > Modify Actor ).
Click the Scale Actor icon.
Set Method to By Points.
Turn on Proportional if you want the actor scaled in all three axes or select the desired axis.
Select the actor.
Enter a data point to define the scaling reference point.
Enter a second data point to define the amount of scaling.

Scaling an actor with Proportional set to On.