Jim,
I'm just guessing because I don't know for certain. However, I would say currently 2,5,6 and 18 are all around 6000sf
4,9,11 and 15 are probably the smallest greens with the 11th being 3600sf or so and the others in the low 4000sf range.
The original green sizes were all significantly larger with the exception of 6,12,13 and 17 which have been recently enlarged.
With expanded greens, the pin positions can be much tougher so I think the course will actually play harder with the larger greens. The positioning off the tee or the second shot on par 5s become that much more demanding as pin positions will certainly influence angle of approaches more than they do today.
Your question about which is more important to decent scoring (a different concept for different classes of player--let's call it par for someone like yourself) is an interesting one.
With the fescue up and typical summer winds, I would say the tee shot is slightly more important and will be even more so with continued green expansions. The position you approach the pins from is critical. This isn't to say that the approaches are not critical as you simply cannot score well if you miss the greens. Yet I don't think Flynn's green designs are so unforgiving that a good putter doesn't have a chance for a two-putt on most greens. But first things first. You have to put yourself in the right position on the fairway so that your approaches do not leave you above pins (as at most courses) and you cannot miss the greens. The large areas of closely mowed grass around the greens leave long and difficult recoveries. The balls don't come to rest on down slopes allowing lofted recoveries. The rough is sufficiently far that the ball will often find a position well below the greens.
In general, the middle of each green leaves you a potential 2-putt. There are few internal contours that make putting overly penal. Shooting at pins, especially near the falloffs and bunkers can result in a whole bunch of others if you do not or are unlikely to execute. The 5th green is a classic example of that. If you miss that green long and left, good luck recovering to a back right pin position. There are lots of recoveries around the greens that are so difficult, many of which are not apparent without repeated play.
So you simply have to put yourself in the correct position in the fairway to approach the greens for an ideal angle of attack if you choose. If you are in the wrong area of the fairway, the center of the green is rarely a bad place to be. But you have to be able to hit a good shot into the greens and have an understanding of where the trouble areas are--the falloffs and slopes into the bunkers. They are subtle and so not readily apparent. It takes time to learn the nuances around the greens.
In summary, I think the positioning in the fairway, being the first shot of each hole is key, but are nearly in balance with difficulty of the approach shots as you would expect in a great championship design. There is a great deal of variety to the tee and approach shots dictated by hazards, fall-offs (some very subtle or obscured), collection areas and fairway contour lines. You have to be thinking all the time.
I guess I'm hedging saying they are both very difficult with a slight nod to the tee shots. What do you think?