The height rule is a rule from anime and manga stating that cute, bubbly females are almost invariably short. This appears to have, at least somewhat, transferred to the PPC and its female Agents, along with some of the males (a good number of whom appear to be shorter than average). This does have some backing in that females tend to be shorter than males, but is also often exaggerated in said anime and manga. A notable PPC exception to the rule, however, is Nat Freidar, who stands taller than most female agents and many male agents at six feet tall.
A variation on the height rule specific to Yaoi states that the Seme must be taller than the Uke. In badfic, this has been known to extend to describing the chosen uke as much smaller than he is in canon, or the seme as much taller, just so they will fit the stereotype.