physiological constants and constraints

I wonder how much discontinuity is created by physical, in particular physiological, constraints?  And I wonder how much those discontinuities are exploited in investment decisions and forecasts.  I'm thinking about things such as body sizes, minimum sleep periods, and elimination functions and their relationship with economies of scale and extrapolation of product sizes and uses.  For example, cars have a hard time getting smaller than a certain limit because people generally fit within a certain band (and so you get oddly proportioned vehicles such as a Smart).