I don't know the actual square footage but several of the greens at Plainfield have very few pinnable areas, both because of their small size and severe slopes.
The first green while not small in size has such a severe slope that there are only two pin positions. This is the case with the 2nd, 10th, the very small 11th, and the 18th. Ross's great greens usually require the golfer to play a superb shot to get the ball close or use the slopes, both on and off the putting surface. Since Plainfield has a pretty substantial maintenance budget and a moderate number of rounds played, wear and tear isn't too much of an issue. I think these greens are some of the best on the course, requiring a lot of thought over each shot.