@Prognathodon saturator 101 Apparently, popular media commonly depicts Cardabiodon as a monstrous 7-9 meter shark that can take down most pliosaurs. However, I do not see any backing for this claim (I've heard from somewhere that this claim was from a documentary about Cretaceous Lamniformes, but as always, prehistoric documentaries are almost always exaggerated in some way for the $$)

As far as I know, the largest Cardabiodon fossil is the holotype  of C. ricki itself. Siverson (1999) estimated its length to be around 5.4 meters after noting that the vertebra from the holotype match the same diameter with that of modern great white sharks of the same length. Siverson and Lindgren (2005) placed the estimates of C. venator to 5 meters.

Dickerson et al. (2012) found that the tooth based estimates for Cardabiodon were underestimated due to that the teeth turn out to be smaller compared to the body than first expected. They concluded that any Cardabiodon with teeth measuring up to 20 millimeters in crown height can measure up to 3.8 meters in total length.

Overall, according to Newbrey et al. (2013), the maximum length of Cardabiodon ricki should be a modest 5.5 meters. Not as big as the gigantic Cretoxyrhina, but still formidably bigger than most great whites. Newbrey et al. (2013) also suggested that Cardabiodon might be quite bulky, if that tells anything significant.

There is no direct evidence of what Cardabiodon ate, just possible suggestions based on size and tooth morphology. In my opinion, it's possible that Cardabiodon could have had a similar diet to that of Otodus obliquus considering somewhat similar tooth morphologies. 

The largest isolated teeth that I have found thus far at the type locality of Cardabiodon ricki are about 15% larger than the corresponding ones of the holotype so I would estimate that large individuals may have reached a bit more than 6 m. The holotype, btw, was not fully grown (see Newbrey et al. 2015). Basing TL estimates on crown height is, imo, a somewhat flawed idea as tooth height for the tallest teeth in the dentition is highly variable in modern lamniforms. Some have very tall anterior teeth (makos) and others have short anteriors (Alopias pelagicus; this also applies even if this species had a more normal looking tail).

Albian cardabiodontids were considerably larger than C. ricki based on the size of their vertebrae. The undescribed cardabiodontid (a bit different from Cardabiodon but clearly still closely related) in the Toolebuc Formation had vertebrae up to 140 mm in diameter which is more than 50% larger than in the holotype of Cardabiodon ricki. It is therefore quite possible, based on vertebral size, that the largest (Albian) cardabiodontids reached more than 8 m in TL.

The anterior teeth of the Albian cardabiodontid, while impressive, are not as robust as those of very large Cretoxyrhina mantelli so my best guess is that it would have been positioned at a slightly lower trophic level. Having said that the modern species with the highest trophic level is the broadnose sevengill shark, which is much smaller than the white shark. The former is pretty impressive as they start feeding on seals at a body weight of just a few kilograms!

The overall trend in cardabiodontids (Albian-Turonian) appears to have been: (1) a reduction in body size (I would estimate from 8m in the early half of the late Albian to perhaps 5m in the middle Turonian); (2) an increase in the size of the teeth relative to maximum vertebral diameter (implying decreasing number of tooth files); (3) a widening and labiolingual compression of the crown; (4) an increase in the relative robustness of the root; (5) a reduction in the development of lateral cusplets.

The teeth of the youngest species, C. venator, convergently started to look a bit like Parotodus.

I wonder if that 'documentary' was the talk I gave at the Royal Tyrrell Museum where I mentioned the 8m estimate and showed pictures of the 140mm vertebrae (kept in a small private museum in Boulia, QL). 

Based on associated vertebrae from the Toolebuc Fm, individuals exceeding 6m in length would not have been a rare occurrence.