Centreboard - Trading length for entrained water

For a centreboard, as opposed to a daggerboard or fixed keel, is longer always better given that it means a longer slot in the bottom of the hull and more water trapped in the board casing?

 

The water in the board casing is not meaningful as long as the board is heavier than water (i.e. the displacement is already there). What is meaningful is the length of the slot to the width and the centerboard seal (if any). Those are the two things that effect drag. Longer narrower slot with seals is better for drag.

As opposed to daggerboard or fixed keel, that is more a stability issue than drag issue. Too many other factors effect the overall drag to say one type (daggerboard, centerboard, or fixed keel) has more or less drag than any other. The type of lift generator has to be designed synergistically with the overall requirements of the vessel.

(edited to answer the OP question).