This painting has always been one of my favorites at the National Gallery. It's quite large, about eight feet tall, and if I recall correctly, hung pretty high so the figure seems even larger than life.
This painting is considered a 'Grand Manner' portrait.
From National Gallery website:
This painting caused quite a stir in 1782 London because up until that point, no 'Grand Manner' painting had the subject engaging in activity. That makes me both giggly and a little wistful.