Length is a minor factor in what might be considered a seaworthy
boat. The skipper
and crew are the greatest factors when it comes to safety
. The design of the boat and its safety equipment
irrespective of its length is the second greatest factor.
There are many relatively safe boats in the 25 to 35 foot range that are suitable for one person. Personally, I would not consider one under 25 feet just for comforts sake. A boat over 35 feet would probably be more than one person needs unless you are one who insists on all the creature comforts. This is just my very rough estimate. Others will have ideas that are probably just as valid.
There really is not an answer for what length is safe.