sorry, the relevant part of the book is in the first few pages of the first chapter, where JBS Haldane discusses the physical limits of mammalian anatomy due to load bearing constraints on bone cross sections and hydraulic implications for a heart that must circulate blood/lymph through a gigantic body, and how all of these and other separate systemic envelopes merge to form what we understand as the "right" sizes for living things etc.
Some portion of this chapter is occasionally excerpted to deflate claims about extraordinarily large or small purported alien creatures, though Haldane was setting up an argument for dynamically limited systems in general.
I thought it might be an interesting skim given the similarities to your comment above.