I did some cursory research, and found that C&O boats ran from about 40 feet to 70 feet, and the locks were 20' or so wide.
The length depended mostly on the creature comforts aboard (For ALL the creatures on board... crew, spare mules, etc.) as it did on the commodities shipped. The Collier were generally shorter owing to the weight.
Trainers and merchandise boats would be longer with more cabin space for larger crews.
I'm also going to selectively compress my canal, so I'm going to start with a beam of 15' and a length of 50', or roughly the size of a modern boxcar.
I'd also like to model a lock.
Lee