I think he is talking about length. You can use your old mainsheet to determine the length. If it lets the boom go out to its farthest distance (hits the side stays) and you still have enough tail to control the line from the positions you usually sit in the cockpit, then it is long enough. If not, then lengthen it accordingly, or shorted it if you have too much line in the way. It is not important to be spot on the spec. It is important to have enough line. For a 30 foot boat 3/8" is the smallest you would want and probably 7/16" is better, but the size of the sheaves in the blocks will determine the max line size it can carry.
