[flatheadmurre]

A to small value can give you arcing.
The capacitor value should be just big enough to absorb the voltage over the point until they open enough so the voltage can´t jump between the points.
A value bigger then needed affects the coil output since the colapse of the field goes slower with a bigger value of the capacitor.
So you want a small value from performance point of view...but big enough to give you a acceptable points life.
If you look at the points after they been run there should have equal wear on both contact surfaces...more metal on one side means the capacitor value is to big or small.