Most textbooks use the static margin (the distance between the aircraft's centre of mass and its aerodynamic centre) to assess the longitudinal static stability.
Instead of using an absolute value for the allowable range of the static margin, it is expressed as a percentage of the Mean Aerodynamic Chord (MAC) - for example between 5 and 30%. Why is this so?
For a given aircraft, the higher its aspect ratio for a given wing area the shorter will the chord become. Hence the MAC will also be smaller. This will in turn reduce the absolute value of the allowable range for the static margin.
Is this simply a convention or is there an aerodynamic or flight mechanics reason why the allowable range for the centre of mass is smaller for smaller MACs or shorter wing chords?