ok
it should be the same diameter as your main sheet and boom vang. the length depends upon the attachment points. I have a seldon boom, which is designed with a preventer attachment point on the clew end. i had u-bolt padeyes installed on the flat area of the toerail just forward of the chainplates. (i attach to these via snatch blocks, but be careful to get the heaviest-duty blocks you can find, because the angle is pretty severe for a snatch block.)with that setup, a 45' long line works fine. Some people use two lines so that they don't have to leave the cockpit after a gybe. I don't.