As Ivan said, you've answered your own question, you can be competitive from 14 -20 stone (in old money)
The rig evolution has, in my opinion, made this possible, and changed the demographic from purely a fat boys boat to a more allround boat. Which is a good thing.
In short I don't think there is an ideal weight and don't feel we need to advertise such a restriction, which might put people off as they think they might be too light or too heavy.
Good general fitness, technique and sailing ability will always win, regardless of actual body mass