Size of Mamenchisaurus sinocanadorum
The holotype specimen originally discovered in 1993 is described as being 26m in length:


  An articulated neck of a large, mature sauropod, with enormously elongated cervical ribs, was discovered in strata of Late Jurassic age in the eastern Junggar Basin, Xinjiang, People's Republic of China. The animal is estimated to have originally measured 26 m in length, but was lightly proportioned relative to Brachiosaurus and Apatosaurus.
  
  ...
  
  In the present specimen, one rib from the midregion of the neck was seen to be at least 3540 mm long in the field, and after preparation measures 4100 mm in length.
  
  
  "A large mamenchisaurid from the Junggar Basin, Xinjiang, People Republic of China", Russell, D.A., Zheng, Z. (1993) Canadian Journal of Earth Sciences
  




The larger measurements come from additional remains attributed to this species, but not yet formally described. Here is the relevant section from Paul's book:




The Princeton Field Guide to Dinosaurs, Paul, G.S. (2010), Princeton University Press.




Paul's figures are echoed in the following papers:


  2006 年，中科院古脊椎所在新疆昌吉奇台（原中加马门溪龙标本发现地附近）发
  掘出一具巨型蜥脚类恐龙，经初步鉴定应归入马门溪龙。从发现的部分颈椎长度推测，
  这具马门溪龙长度可达 35 m 以上，成为了中国乃至亚洲目前发现的个体最大的恐龙。
  
  
  
  
    [Google Translate]
    
    In the year, the ancient spine of the Chinese Academy of Sciences was located in Changjiqitai, Xinjiang (near the original site of the discovery of the Jiamamen brook specimen).
    
    A giant sauropod dinosaur was excavated and the initial identification should be attributed to Mamenchisaurus. From the length of the cervical spine found,
    This [Mamenchisaurid] can reach a length of more than 35 m, becoming the largest individual dinosaur ever discovered in China and Asia.
  
  
  
  
  
  马门溪龙化石研究综述, (2008)
  





  Mamenchisaurus sinocanadorum (体长26~35 m)
  
  
  Advances in research on dinosaur gigantism, X XU, Q ZHAO (2016)
  


Though the first paper does not cite a source, and the second paper cites only Russel and Zheng's original paper describing the 26m specimen.