I'm talking off the top of my head here - but it's not against the rules. I can't see why length of plane affects stability. The greater the distance between cg/cp and the rear stabilising assembly, the greater the stability - other things being equal. But there is no reason why they should be equal. I would have thought Boeing or Airbus would adjust tail surfaces for each model in order to optimally meet stability targets.
Longer planes are probably more sensitive to attitude control as they seem to have the same size of landing gear and thus have more limited allowable pitch ranges.