http://shop.s2owners.com/cordage/cordage.htm
If you select an S2 with a similar mast height ("I" value) you should be good, otherwise just call the site and they will help you out.
A little long is better because you can cut it but you can't make it grow. I ordered some line for a halyard (boats have lines, sheets and halyards...but once they are destined for a boat they graduate from rope) that turned out to be about ten feet shorter than I ultimately would have liked. Which stunk, because I later didn't have enough to reach my sheet winch in the cockpit when I ran the lines back, because there was a 90 degree angle the way the blocks were set up. That boat didn't have a winch on the cabin top.
Whether or not the lines are run back to the cockpit and how (directly back or to one side and then back) impacts the length of your halyards. If you will need to raise the sails from the base of the mast then the halyards will be shorter, a little more than twice the I value, but if they go to the cockpit that number will be closer to 2.5-3X, depending on the path they take, plus enough for a few wraps around a winch (which really shouldn't be a necessity on a boat that size).