DC regs state that the leash can be no more than 4 feet long:
“900.3 No person owning, keeping, or having custody of a dog in the District shall permit the dog to be on any public space in the District, other than a dog park established by section 9a of the Animal Control Act of 1979, passed on 2nd reading on September 20, 2005 (Enrolled version of Bill 16-28), unless the dog is firmly secured by a substantial leash. The leash shall be held by a person capable of managing the dog.
900.4 The length of the leash required under § 900.3 shall not exceed four feet (4 ft.).”
It’s an old reg — it seems like the standard leash you find in stores is 6 feet. I had a hard time even finding a 4 foot leash when I wanted one to train my dog.
I’ve never seen anything definitive about retractable leashes, so since they aren’t specifically outlawed, it would appear they’re legal. But the language of the regulations seems to state that if the leash is let out more than 4 feet it could be considered a violation.