RJ,
I do have those statistics in mind, but I don't let them drive me nuts! They are a starting point, one of a number of criteria (number 46 out of 87 according to Jeremy, perhaps a bit higher with me), but not a strict rule!
I usually size greens for the bogey player who will predominate play on any given course, and that distance and accuracy equation probably hasn't changed a bit. Those stats for green size were based on 2/3 of all 20 handicap (or scratch in another tabulation) players hitting a target of a specific size. That seems like a reasonable goal to me. Hey, would you want at least 2/3 of the group in front of you to hit the green, or not? Perhaps you prefer to stay out there all day long!
Although I've posted this before, I have roughly summarized the USGA chart for the target size (shamelessly rounding for easier "in my head math") for the 20 handicapper according to the slope rating as:
Width - 40% of approach length yards, in feet
Depth - 60% of approach length yards, in feet
i.e. an "average green" for a 100 yard approach is 40 feet wide and 60 deep, for a 150 yard approach, 60X90, and for a 200 yard approach, 80X120.....
Again, these standards are said to be derived from field study, ie watching a gazillion golfers hit shots to typical holes, in all kinds of conditions. Nonetheless, I often adjust each green size using common sense - like making it wider in a predominant cross wind hole, for instance....
For scratch players, the width and depth is about 33-35%, ie a 210 yard shot requires a 70X70 target.
For the superintendents doing the maintenance math out there, you will probably note that the smaller sizes for scratch players don't begin to give enough useable cup space, another good reason to design for the bogey player!
I often size the big middle of the green to bogey standards, and size the Sunday pin location to scratch standards, accomplishing the best (I hope) of all worlds. That is, of course, unless I decide just to do a small, single target green to instill the fear of God in all, or a huge triple tier green for the "green within a green" concept. I do each once in a while, and also throw in the occaisional "conversation piece" green that makes you go "huh?"
What's interesting is that while the green sizes vary, the shot value, if defined as some have here, relates to the difficulty of the shot, is exactly the same! There is a linear relationship to room required for hitting a 200 yard shot versus a 100 yard shot 2% off line, and it is exactly proportional to distance, according to the usga.
As an architect, I have no problem asking the player to hit, say, one long approach with more accuracy than "average" and one with less, if it fits in with the hole. Same with short irons, par 3,4, and 5 holes, etc. But generally, most targets should be big enough that most players can hit it with reasonable consistency, or the course will develop a reputation for being unfair. As Jeremy says, most shots should probably be in the "average" category, if for no other reason to make the easy and hard ones stand out!
As you say, the key is not to force this on the topography. If I find a long, narrow green site, this immediately suggests a green that places a premium on direction, as it will probably be narrower, but perhaps a bit longer, than USGA stats say they "should be." A large open area obviously suggests a bigger green than normal, etc. From time to time, a contoured green site presents the opportunity to do something really unusual and natural, and those usually turn into the concept, or conversation piece greens.....
For most average sites, and most courses, form should follow function, and if the green's function is to recieve a long iron shot, it should be designed to do this. In this case, size matters! You have to start somewhere, and if you start with size, which is easy to grasp, you adjust other details. If you start the green design with using a particular natural feature, you adjust other details, plus size, to match whatever your most important criteria is.
For any given green site, hundreds of architects could give you literally hundreds of design options that accomplish this in different ways, so there is no formula. But, simple physics and experience dictate the green should be sized roughly in proportion to its intended function, and there is no good reason it shouldn't be, IMHO.
BTW, the USGA says 32 yards is about the mimimum a scratch player can consistently hit a fairway, although their championships often feature narrower fairways! Regardless, this suggests to me that a fairway ought to be at least 16 yards wider than the minimum, or about 48 yards to encourage play to either side of a fairway for strategic value! But, that's another topic!