I agree that mainsheet is best done with an eye splice at the becket at the boom block.
If you haven't done an eye splice, its a good learning experience. They're actually not all that difficult to do.
Jibsheets at about 1.5 times the boat length would be a general rule of thumb, and probably about 3/8" diameter.
If you are using a Gennaker, lighter line would be better because the weight of a heavier line tends to cause the sail to collapse more readily in light air conditions.
There are many differing opinions on how to secure jibsheets to the clew. My personal preference is a bowline. Tied correctly, with a short tail, it will remain secure and still be easy to untie.
As a rule, I have separate sheets for each headsail.
If you can get your hands on the owners manual for your boat, there is a page with specs onthe line diameter and type for every line on the boat. As I recall, much of the line on my boat was specified as StaSet in varying colors with some of the sheets and halyards the identical diameter but in different colors. If color is a non issue, you could buy a 600 foot spool of line and replace sheets and halyards at the same time.
Just my 2 cents ...