(EDIT: Updated version, find it here: I, SUPERMAN)
This was done piece by painstaking piece using an inordinate number of reference images.
I started by looking through GIS, but (eventually) noticed that a lot of the hits were fan artworks that weren’t 100% faithful to the look & construction of the Giant as he appears in the film.
Eventually, what I wound up doing was using stills from the DVD when I could find an angle of the Giant’s body close enough to what I wanted—for example, the torso chassis is based on a few stills I pulled from the junkyard scene where the Giant assists Hog Hug in his blast-off into space.
When there wasn’t a good enough reference angle in the film, I had to get more creative. With the forearm and fist, for example, I was able to find a “close enough for government work” 3D model for Google SketchUp, which I was able to rotate around and manipulate until I had myself a reference shot I could use.
The finer details, however, weren’t really that fine, so for things like the complex ball-and-socket joints where the hip meets the leg, it just involved a TON of watching, pausing, rewatching and so on until I had an intuitive feel for how the pieces were shaped and fit together in three dimensions.
This was a labor of love from which I would settle for nothing less than perfection. All in all, from start to finish, it took 4-6 weeks between February and March 2011. Some days allowed for 3-4 hours of working & reworking, and there were other days where I wouldn’t have other work to do and my obsession would pull me deep into the rabbit hole, eating up 12 or more hours of work in Inkscape, broken up by meals & research. (This of course includes about a few days wasted on work based on some of the inaccurate fan art mentioned earlier.)
If there’s enough interest I may put some kind of step-by-step progression of everything up on Tumblr.
(First sold 5/20/12 — Thanks aquinas30!)