The answer depends on the specifics of the design in question. Up until now you have been talking about a heavy 25 footer. By that I assume that you are considering a boat in the 4,000- to 5,500 lb range. Normally, you would want the keel and bulb of a moderate draft fin to weigh somewhere in the neighborhood of 30% to 40% of the boat, or in other words, 1,200 lbs to 2,200 lbs.
I would think that you would want something in the neigborhood of 4 to 4 1/2 feet of draft in the fully lowered position. The canoe body of the boat will probably extend roughly 16" to 18" below the waterline with a vee hull form. You will end up with something in the range of 2 ft. to 2 1/2 ft. or so of draft in the fully raised position (you won''t be able to sail with the board raised that far) and I would think that you are probably looking at a dagger board that would be roughly 2 1/2 to 4 feet long (horizontally) depending on whether you want to be able to sail with the keel in the partially raised position (perhaps 3 feet of depth would be a practical minimum). The shorter (in the horizontal dimension), the less likely it will be that the board will jamb or damage the trunk when you run aground or raise it in the trunk.
I should note that you are now in a territory (heavy displacement, cruising catboat, with a daggerboard with a bulb) where no designer has ventured before but that is not to say that it is not an idea without some merit. (Again you would be way ahead of the game if you scale back the weight and the weight up high a bit.)