I think an "average" sized green today is around 6,500 square feet, but I would guess that some architects average 5,000 and some average 8,000.
My idea of a small green is under 4,500 square feet. You can do those all day on a private course that isn't going to get much play, but if you're going to do one on a highly-trafficked course, you'd better be sure that nearly all of it is cuppable, that there is good air movement and good sun, and that traffic flow on and off the green is not hindered by too many bunkers [otherwise, parts of the green around the corners of the bunkers which you need for hole locations are going to get worn out from traffic].
The smallest greens I've ever built are right around 3,000 square feet -- the 5th at Sebonack and the 9th at The Rawls Course come to mind.
Mr. Nicklaus is a big fan of small greens. In our first conversation about Sebonack, he asked me if I knew what was the biggest green at Pebble Beach? It's the 18th -- and it's only about 4500 s.f. A couple of them, including the 8th, are well under 3000 s.f.