>>19850660Birds are measured by length in addition to wingspan. Many field guides only featured length for most non-soaring birds until wingspan started to be included by some top-of-the-line guides. Strictly speaking, birds are also measured by mass, bill length, head length, tibiotarsus length...
Source: am birder with biology degree
Plus the whole idea behind most of the birdmons is that a standard-sized kid could ride on the fully evolved form. A height of around 4-5 feet makes that possible.
In Pokemon, the rule of thumb for the (often bullshit) measurements is:
Biped: Height, toe to top of head
Serpentine: Length, nose to tail tip
Quadruped: Height, to either top of head or shoulder, no one really knows
