How can you cruise only half the Panama Canal?

Cruising down the Panama Canal has to be on the bucket list of every serious cruiser out there, normaly from Fort Lauderdale to L.A or vice versa.
For people who either don't want to or just don't have the time most cruise lines also offer the option of only cruising half the canal so you can say you can experience it without it taking up a good chucnk of your holiday.
But here's a question for you. In a great big cruise ship, how do you cruise only half a canal, it's not like you can turn around half way through?
Is it?

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).