In horners syndrome, does the miotic pupil remain a constant size or does it also respond to variations in light level?

It still responds. In Horner's syndrome the pupil can still dilate to varying intensities of darkness but just not as forcefully or briskly as the normal eye. Another way to put it is that depending upon the severity of the sympathetic pathway interruption pupillodilatory muscle activity can be affected anywhere from mildly to severely or even paralyzed. Only seen dilation paralysis a few times but it can happen.