Re: what is the biggest sauropod in the history of the planet

It is hard to know exactly how large the largest sauropods were because 
we only have a few examples of some of the species (with no way to tell 
if they were large or small compared to others of their kind) and because 
so many of the fossils are missing parts, particularly tails.  Some of 
the very largest examples are estimated based on comparing the size of 
single, detached bones to the same bone in another, similar sauropod¸ 
then just scaling up .
Early sauropods were small - maybe around 1.5 meters - but over time, 
they became larger. Some of the largest sauropods like the Supersaurus, 
Diplodocus, and Argentinosaurus reached 30–40 metres in length, and 30-50 
tons or more in weight.  Other large sauropods include Saltasaurus, 
Opisthocoelicauidia, Rapetosaurus, Paralititan, Bruhathkayosaurus and 
Amphicoelias.  Based on a single 2.4 meter  vertebrae, Amphicolias may 
have been as long as 56-62 meters.  Weights for the largest sauropods are 
also estimates since we don’t know the exact sizes and we don’t know the 
exact shapes of their bodies, however, some estimates are as high as 150 
tons or more.

References:
1.  Biggest of the Big: A critical re-evaluation of the mega-sauropod 
Amphicoelias fragillimus Cope, 1878, Carpenter, K., in Foster, J.R. and 
Lucas, S. G., eds., 2006, Paleontology and Geology of the Upper Jurassic 
Morrison Formation. New Mexico Museum of Natural History and Science 
Bulletin 36., https://scientists.dmns.org/sites/kencarpenter/PDFs%20of%
20publications/Amphicoelias.pdf

2.  http://en.wikipedia.org/wiki/Sauropodomorpha
3.  http://dml.cmnh.org/2004Sep/msg00086.html
4.  http://news.bbc.co.uk/2/hi/science/nature/4031789.stm
5.  Weischampel DB, Dodson P, Osmólska H, "The Dinosauria", University of 
California Press, 1990