The video below shows Charlie over at BrickVault building an extremely impressive custom model of a Star Wars AT-AT at about twice minifig scale. The model was designed by Peter Brookdale, aka CaveGod2009 on Flickr, and was exhibited at various shows over the last few years.
Peter started the build in January 2010 and documented his progress and shared stories with people on the EuroBricks.com forum while he built it. At one point commenting that the ankles had a breaking weight of 12 lbs each!
This version of the AT-AT model required over 6,000 pieces to complete, which were ordered from BrickLink.com. As detailed in the video, the body and head are so heavy they required modifications to the legs to be properly supported. I can’t imagine the size and weight of this awesome model.
The set below is listed on Amazon.com, and really pales in comparison to the complexity and enormity of the project in the video above. That said, unless you’ve got the pieces or the $800 budget to build this (and somewhere to put it), the classic set 75054 might be your best bet.