Let's not get too attached to the idea about fin length, as it's only part of the picture. It's really aspect ratio that tends to define a fin's outline, and ultimately its length.
Sometimes it's really more meaningful to think about a fin's total area. For example, I have a True Ames 24cm low aspect wave fin that roughly has the same total area as my high aspect Tectonics 34cm Goldwing slalom fin. Although the fins differ in concept and performance potential, I find that they are quite interchangeable on the same board and sail.