Well, a recent paper by the late Elizabeth Nicholls gives an estimate of
the
largest _Shonisaurus_ species, to have been 23 meters long (which by my
recollection of the specimen itself, is possibly a conservative estimate),
and there have been vertebral centra which were much larger than the ones
found at that particular _Shonisaurus_ site.
It is interesting though, that you mention what sort of microcommunities
would have been surrounding an Ichthyosaur fall. At the Sikanni Chief River
site in Northern B.C., where the massive _Shonisaurus_ specimen was
unearthed, the amount of bivalves found in association with the specimen
were significantly larger than the concentration of bivalves in that strata
nearby.