We report on the size variability of coccoliths--calcite platelets produced by planktic marine haptophyte algae--in globally distributed Holocene surface sediments. The sizes of 400-600 coccoliths in 51 Holocene deep-sea carbonate ooze samples were measured using automated scanning electron microscopy and image analysis processes. The resulting coccolith size histograms are highly variable, but the largest 10% in each sample showed a size increase from the tropics to subpolar regions. This is the opposite trend from the one observed in planktic foraminifera, which have their largest tests in tropical regions. In a subset of 13 samples, which cover the major environmental gradients of today's surface waters, coccolith sizes of the nine most common genera were analyzed. These show that the observed macroecological size variability, which is related to a complex mixture of environmental parameters, is mostly the result of changing species occurrence and abundance (biogeography), rather than size changes within genera and species. This Holocene calibration will help to test evolutionary hypotheses of environmental selection in marine phytoplankton and can serve as a useful benchmark for analyses of coccolith size variability in older deep-sea sediments. Abstract Copyright (2012) Elsevier, B.V.