Mainsheet tackle
3/8 line is more than strong enough but it can be tough on the hands if you adjust trim a lot. I prefer 1/2-inch line for sheets because I have big hands and it just feels better.As far as length goes, run the boom as far out as it will go and measure the distance from the traveler car to the boom bail. Multiply by five and then add some extra. If you plan to use the mainsheet tackle to raise/lower your mast, you will need even more line so you can stand at the mast base. Good luck.Peterh23 "Raven"