Do you not have the "Curve" panel at the bottom? This allows you to graphically adjust the curve. Here's how I make the default particles shrink to 0:
Set the Scale to 10 (or some larger value than the default, so the effect is more apparent).
In the "Scale Curve" property, click on "New Curve Texture". Click on the new curve texture and the panel opens at the bottom. The curve starts as a straight line from 1.0 to 1.0, so drag the right-hand end of the curve down to 0.
Note that the scale curve is a multiplier, so you have to set the actual scale first, and the curve will multiply it between 0 and 1.