I would also think the 6 HP would be adequate for the boat size ( I had a 4 long shaft on my 23' and that served fine...), but I would get a long shaft as outboards have generally marginal prop depth, and deeper is, well, always better. Also note that the 4 cycles are bigger than the 2's, be sure to check fit before you are commited.
Here's a link to a discussion thread about outboards and Cd 25s, while the CD 25 is a different boat I would think you would find it helpful to read the comments of these owners of a similar sized boat:
Cape Dory Boats - View topic - best 4 stroke and horse power outboard for 24 ft CD