Just changed it on my daughters laser, from what I found in other postings on this forum, 45ft is about the lenght recommended. For the diameter it seems to vary depending on appication, racers seem to like 7 mm dia. but i will leave the true Laser experts recommend the proper diameter. We got 8mm.
My mainsheets are 46 feet. When you're running by the lee the situation may develop where you want to ease you boom out past 90 degrees. In that case you will need all the extra length that seems to be useless spaghetti in the bottom of your boat.
As far as diameters are considered... my regualr mainsheet is 6.5 mm. However, I do also have a heavy air mainsheet that is 8mm. It's all personal preference but the thinner the mainsheet the better it will run through the blocks off the wind.
Ok, I'll admit to running a 52' (15.5m) mainsheet. Having it this long permits me to let the boom go completely forward, which can be quite handy when sailing square down wind in narrow channels in 25+ knots, as you just drift down wind.
I run a 6mm polypropylene (8 plait), super cheap but it twists as much as any other mainsheet I've used including the Rooster mainsheets. My partner uses the same mainsheet, except it's 8mm thick, as it is easier on her hands.
> What is the recommended length of the main sheet line?
I run mine quite long, around 50 ft. Really helpful in super-light air (we get a lot) when you need to get the boom well past 90 deg to stay by-the-lee without some bizarre yoga pose to manually hold the boom out. Also fast to go beyond 90 deg broad reaching in some conditions. Also just like to play the game of "how far by the lee can i go?" (turns out close to beam rach on the wrong side ;-) In my experience anyway, 45 ft is just too short.
Currently using 5mm diameter floaty stuff -- same reason, light air -- keep the sheet out of the water. Also plays out faster than thicker line. But admit is is pretty nasty on the hands in a blow.
Hint: it is much easier to cut a rope shorter than to cut it longer. Start longer than you think, try it for a while in various conditions, then shorten if it really is too much.
To add to Alan's post, being able to allow your boom to go all the way forward is hugely helpful if you are forced to make a downwind approach to a beach or similar scenario. Once you are close to where you want to be simply let the mainsheet go and you are done!! No worries about trying to round up and and perform some kind of fancy octaflugeron to get the boat to the beach!!