How big can they really get?

Several customers of mine have just come back from the Oasis of the Seas, and yes I was very jealous, as I can’t wait to get out and see this ship for myself. For those of you don’t know she’s enormous, the biggest cruise ship currently at sea. Speaking to one woman she told me she couldn’t believe how big it was and couldn’t imagine anything bigger.

Which got me thinking;
How big can they go?
Is there anything that limits their size?
Are there plans for anything bigger?
And most importantly do people want bigger? (Yes, yes, yes from me, I love huge cruise ships).