I think I see the problem - you have this idea that all cells have a hardwired limit on the number of divisions. While it is true that there are a number of mechanisms in place to control cellular division, these mechanisms are flexible and vary between cell types. For example, the basal layer of the skin and most hair follicles continue producing cells throughout an organism's life - there's no limit there! The bone marrow is the same. Modern cell biology assumes the presence of "stem cells" throughout the body in different organisms to help regenerate tissue in the presence of injury. The fact that I can't regrow my arm has nothing to do with cell division limits, since I will produce more than enough cells to replace the lost arm mass in my lifetime, but there's no way to get all of the right stem cells in the right place with the right signals.

In any event, talking about limits on the number of cells is very different from talking about limits on cell size. Furthermore, limits on the number of cells are controlled by genetics and there are no fundamental limitations on cell number beyond macroscopic limitations on organism size - things like energy needs, gravity, and evolutionary fitness. Cell size and cell number limitations are not preventing us from growing a larger heart - this is limited by genetics and fundamental limitations in scaling up our body architecture. Similarly, insect size is limited by things like oxygen diffusion and the structural integrity of their exoskeleton - they can't grow much larger because their exoskeletons can't support the increased weight, not because there is any fundamental limit on the number of cells.

In your critique, you suggest that somehow the size at which a whale fetus's heart starts functioning is important. I'd say that this is completely irrelevant.