The load on the halyard can be roughly approximated as twice the vertical load on the mainsheet. There's actually more to tha calculation but that's close enough for government
work. Add 10% for every pass of the line over a block.
The total load carried by the mainsheet is divided between the number of parts
plus 10% for every block. If you have two blocks on the boom, with a total of four lines to the traveler, the point load is divided evenly between the lines. In theory one should should divide that by the cosine of 1/2 the angle between the two blocks on the boom and the fiddle block carrying the lines on the traveler but that usually doesn't make much difference given the fact that the safe working load of the line is only 20% to 25% of the breaking strength. For example, if the theoretical total mainsheet load is 1200 lbs and you've got four parts
attached to the boom, passing over two sheves, each "part" is carrying roughly 360 lbs = [(1,200 x 1.20)/4]
Not a heck of alot of load in the grand scheme, eh? In such cases one chooses line based on what's easy to handle verses the loading, with 3/8" being about a minimum.