The most common lengths are 8” or 10”. There can be many factors determining which is best for you but usually it comes down to which is more important in your application speed or power.
The 8” version is designed for speed. With a smaller arc to go around, you can get more rotations at the expense of the greater leverage. This makes it the right winch handle for smaller boats, especially racers who are looking for a quick way to grind in or a jib or genoa or even on a larger boat in lighter breeze. Since it is shorter in length, the 8” model also is better for cramped quarters seen in the cockpits of some older race boats or for running backstays. Very few crew members or skippers I know appreciates a shot to the kidney with a winch handle, in situations where this could occur an 8” winch handle might be the answer.
The 10” is designed for power. With a larger arc, it generates more power per rotation however you will be turning the winch more slowly. For larger boats with larger headsails especially in big breeze the extra leverage of the 10” handle will be very welcome on most boats. Many cruisers and racers will find this the right handle for the job and especially well suited where space is not an issue so that you will be more effective with each grind.