The Y / Div value defines the spacing between vertical divisions (denoted by graticule lines when the graticule is on). Increasing the value of Y / Div reduces the height of the trace; decreasing it increases the height of the trace. Note that doing so also changes the values of Y Top and Y Bottom, because there are always 10 vertical divisions, whether the grid is currently visible or not. You can turn the grid on and off from the Display Preferences: Trace tab.