Display scale sets the height of the curvature graph above the surface. Note that if the curvature is 0 (straight) then the graph will be on the surface. I typically start with a value around 100 or slightly larger.
The only directions available for curvature graphs on surfaces are U and V of the surface.
I usually use the buttons to the right of where you can type the scale value. If the graph is too small or appears to be just a white line on the surface (or curve) click and hold the increase button (right pointing triangle). If the graph is too large click and hold the decrease button (left pointing triangle).
Curvature values can vary be several orders of magnitude, even on a single surface. So the appropriate value for display scale on one surface or part of surface may not be appropriate elsewhere. No single value is appropriate everywhere.