I'll try a short answer
Its a moment arm relationship that sets the size.
Consider the thrust loss for a outboard wing engine vs the distance ouboard from the fusealage. Now size the fin-rudder combination to handle this at the slowest manuevering speed.
Since to control the moment of an engine out you need aerodynamic forces against the fin and rudder, then you size the rudder stiffness and attachment strength to keep the stresses within acceptable range.
As engines increased in thrust so did fins increase in size. As fins increased in size and thus aerodybnamic forces their stiffness and stresses remain about the same due to clever structural design.