Size and Weight:
As sauropods go, Shunosaurus wasn't even close to being the biggest--that honor belongs to giants like Argentinosaurus and Diplodocus, which weighed four or five times as much. What makes Shunosaurus truly special is that paleontologists have unearthed not one, but several, complete skeletons of this dinosaur, making it the best-understood of all the sauropods, anatomically speaking.
Otherwise similar to its fellow sauropods (especially Cetiosaurus, to which it was most closely related), Shunosaurus distinguished itself with the small club on the end of its tail, which it likely used to swat away approaching predators. There's no way to know for sure, but the reason bigger sauropods didn't have this feature is probably that the tyrannosaurs and raptors of the Jurassic and Cretaceous periods were smart enough to leave the adults in peace.