Tom Doak,
Are Cypress Point (6,524), Merion (6,592), Fishers Island (6,615), or Crystal Downs (6,518) considered "short" now because they are under 7,000 yards? Or are they considered "just long enough" because they are over 6,500 yards?
Personally, I don't think White Bear Yacht Club is any better or worse because it's "only" 6,471 from the back tees and I think if you started added back tees everywhere it would become too difficult for all but a handful of players, so what would be the point of that?
To me, many "long" courses of over 7,000 yards can get really boring because in order to reach that length they have to sacrifice variety. How many "Championship" courses have a par-3 that is under 120 yards from the back tees? Few, as on courses like that they all seem to be at least 160-170 yards and there are usually one or two 200 yard par-3's. There might be a "short" par-4 of 360 yards, but they are all likely going to be 400-450 yards long.
I don't think there is any magic length for a Top 100 course, but I know what kind of golf course I enjoy playing, and that's one with a lot of variety. If you can have par 3's that range in length from a short 100 yard pitch, to a 250 yard brute, that is neat and I believe makes the course feel like it plays long. Same goes for par-4 holes, which you can have a few short, almost drivable holes coupled with some long 460-470 yard holes that might play short for long hitters, or might be 3-shot "par-5's" for the resort player. As for the par-5's, I think they are always more fun (and strategic) when they are reachable. So if you are building a resort course I think it would be fun to have maybe one true 3-shot hole, and maybe two shorter par-5's that can be easily reached (~500 yards from the back tees)?
If it's a great site, and there are a number of great holes, I think Cypress Point, Crystal Downs, or Fishers Island shows that the rankings will take care of themselves.