Standard mainsheet is 100' of 3/8" Sta-Set polyester double braid on the 6:1 purchase.
We have 88' of 3/8" Marlow Lionbraid, also a poly double braid, but with a tighter cover than Sta-Set (not something I'd try to splice myself). It has held up for 10 years, but is due for replacement and not available locally. Whatever you choose make sure the line is easy to hold and runs well through the blocks.
You can color code the mainsheet, but it lives outside and will fade in the sun, so white with colored flecks generally is best. Splice on both ends so you can end-for-end the line each year to account for the wear from the cam cleat.
We get away with a shorter length since we also use a mainsheet fine-tune.
