ask Paul Taylor, he may help you.

p

Your question is a fundamental and the answer depends on an understanding the local/regional palaeogeography, water depth and energy levels. By subaquatic, I assume that you looking at sediment buildup and transport. My experience is that millimetric packstones and grainstones  can be accumulated downslope over 1-2 kilometres. Bioturbation if likely can speed up the process of fragmentation.

Trust this encourages more debate Richard

I think hydrodynamic regime and  substrate  along with genus are the key factors rightly pointed by André Klicpera.

I can suggest - Dr. Paul D. Taylor, NHM London and Dr.M.A. Sonar  from Institute of Science, Aurangabad, India may give you the answer.  

Maceration, picking and observation under microscope and SEM  these are the routine methods to understand as I understand. May have more advance metohds, someone who know it please add here.