I just finished writing a semeter-long strategic analysis of Royal Caribbean (fun stuff!), and all indications are that cruise ships will continue to grow. There are significant economies of scale in larger ships and pax like the extra amenities.
Carnival, Princess, and Royal Caribbean have all captured and lost the "world's largest passenger ship" title over the last five or six years. Cunard (owned by Carnival) became the current champion when the Queen Mary 2 went into service, but Royal Caribbean has placed an order for a 160,000-ton Ultra Voyager class ship to top the QM2 in 2006. Although I didn't find any formal announcements, I imagine that Carnival will follow suit. The upper limit to ship size is most likely port infrastructure. You can shuttle pax on and off while anchored in a habor, but restocking a cruise ship by tender is very difficult.
[Edited 2004-05-22 02:34:25]
Keynes is dead and we are living in his long run.