We have conducted a preliminary analysis of the history of size change of coccoliths through the Jurassic and Cretaceous. This is essentially based on a compilation of literature data. The results demonstrate that the average size of coccoliths has increased from early Jurassic through the Santonian, stabilized until the Campanian and decreased during the Maastrichtian. Remarkably, this size history parallels the diversity (species richness) history of the Mesozoic Coccolithophorids, and constitutes an illustration of Cope’s rule . The amplitude of change of the average size through time appears to have remained small, which may result from competition by other, larger, contemporaneous calcareous nannoplanktonic groups.