This resource has been provided by Kr00za, scratch-builder extraordinaire, much of whose work can be found on The Waaagh Forum the premiere Ork site on the web. Many thanks to Kr00za for giving us permission to use his work.
Kr00za has already built several 40K scale Gargant models, during the course of which he has established, with a mixture of research and careful consideration, the relative sizes of the various classes of Gargant.
The chart shows all the major classes, plus some other Ork constructions, and includes a War Hound Titan for comparison.
This is the latest version, updated on March 2nd, 2009