Emperor Class Battle Titan.

The 4,000 pt monstrosity has actually been built, used, and most importantly... Photographed.
53" tall(4'5"), weighing ~10 pounds of Plasticard and foam.

http://www.belloflostsouls.net/2010/02/40k-showcase-imperator-titan-must-see.html

Now I REALLY want one..

pretty cool... but I've seen better imperator titans. It looks cool until the top, the cities of death frame used for the cathedral on top really stands out. Like a walker with a church tacked on. In the old art, it was much more integrated. Purely a design problem though. These projects prove how no other tabletop hobby will ever top 40k for being the biggest. Whats that warmachine? Your new 'jacks are bigger? WE HAVE "WARJACKS" THE SIZE OF A CITY, BAM.

Why is it carrying a city on its shoulders?

Read the fluff HC

I used to use one in Epic 40k. It could transport a entire Imperial Guardsman Company including attached commissar and 2 Rhino transports (one for the HQ and 1 for the commissar).

I once played in a game with one of those empator titans, it was an imperial one however.

The best part was, on turn 1, 2 Eldar Flyers each carrying a few sqauds of Fire Dragons swooped down next to it, and after 36 melta gun shots, it was dead, didnt have a chance to shoot.