It is my understanding that "mammoth" plate cameras are those 20" x 24" (50 x 61 cm) and larger. More of a class size than a specific format. However, according to an older edition of the Focal Encyclopedia of Photography, at one time any plate larger than 10 x 12 was considered to be "mammoth". Google can bring up some amazing stuff sometimes.
Carlton Watkins used a mammoth camera to photograph Yosemite in last quarter of the 19th century. Wanna haul that beast around on a bunch of beasts of burden?