I have come to enjoy green sizes that do not necessarily fit the hole. For example, large greens on short par 3s or small greens on long par 4s. I think I like these holes so much because of their rarity and therefore, variety they add to the course. I agree with the comments for variety in size, but I would add variety in green size relative to what you would expect on a certain type of hole.
Joe, I agree and often put my smallest green on the longest par 3 and 4 holes. It tests long iron play for good players, and generally, I believe most average players will be hitting a wedge from just short, so no real penalty.
And, when starting from scratch, I usually separate out the par 3, short 4, long 4 and par 5 holes, and make sure each type gets at least one small green and one bigger than average as an attempt to get the variety you are talking about.
In general we tend to associate small greens (usually round) with older courses. But, in reading all the old dead guys, I recall a quote something like "greens shouldn't be too large.....10,000 SF is more than enough! I believe those guys liked 7000-8000 foot greens and they later shrunk. All in general, of course.