Mike,
There are rules of thumb for green size, if you follow the USGA slope rating guide, although they do present problems. The general gist is that "proper" green size is a width 15% of approach shot distance. Depth for average players is about 22.5%, for scratch players, its about 15%, because they have better distance control. Thus, if you anticipate a 160 yard approach (which is problematic in itself) then the green should be approximately 24 wide X 36 yards deep to accomodate most players.
That statistically argues for greens longer than wide, as Tom suggests. It also suggests side bunkering, since that affects all players about equally, rather than front bunkering which kills most average players, as a general rule of design.
However, nothing is wrong with greens smaller, larger or anywhere really, since variety is key. My only complaint on a course would be if too many consecutive greens were "too small" to hold an approach shot.
As someone mentioned on the other thread, wind, uphill and downhill lies and shots, and perhaps severity of surrounding hazards should all be considered in basic green dimensions.
Tom's ideas about fairway behind the green sort of mirror those of George Thomas, who advocated more fair green behind the target to make an overplay an easier recovery than a short shot would have. Thomas also sized greens according to wind - figuring they could be narrower downwind, which tends to straighten shots, and presumably wider in headwinds.
Unlike Tom, I don't fit - at least initially - to my eye. I have found my eye tends to make greens to small, and I also know we are all creatures of habit, so eye size greens can tend to be too similar, at least in my case. So, I rely on at least starting dimensions off a plan to get things the way I want.
Some may say its formula, but actually, I find that having some kind of formula for variety - like long par 4's with small, medium, medium large and large greens - has improved my designs, not hurt them. While many of us remember greens by features - "the one with no bunkers" for example, I find better players tend to remember things like "the hole with the long iron to a postage stamp."