There clearly is a demand for smaller ships, as evidenced by the responses here. Like many who posted above about 50,000GT and 700 pax is about the max for me. We've cruised so much that rather than go through the lines and crowds a bigger ship involves, we'd just rather stay home.
Problem is, no matter how large they build a ship, it seems to have only ONE reception desk, excursion desk, and entry/exit point. Ive seen lines waiting to re-board a mega ship at a port stretching for blocks. The heck with that.
Silverseas and Seabourn are both planning new ships. Details have not been released, but both have announced that 500 pax is about the most that can be accomodated on a ship that gives a true luxury experience. I'd take that one further, and say that generally around 1500 pax is the most that can be accomodated on a ship that gives a DECENT experience.