Bergmann's rule? I don't know of an analysis; I suspect Bergmann's rule didn't have much of an effect in the Mesozoic because only in the polar regions it was actually cold.
Are they? The biggest known ones are from pretty low latitudes -- all those Morrison and Tendaguru beasties, for example. And while mammoths were considerably bigger than the average living African steppe elephant, the largest known terrestrial mammal is Paraceratherium, which lived in rather low latitudes.
How does this all depend from the sizes of herbivores?