I would examine why it broke. If there's chafe, consider remedy.
I'd use the old length, if it worked. My Harken 1 furler just uses a knot in the line through a hole provided in the drum.
You can use almost any line, but some replacement line is just too thick for the drum. Use smallest diameter comfortable in the hand.
Consider single braid ("Regatta braid" at NE Ropes). It doesn't kink and works well for me. But any good quality line is fine, high strength or super low stretch not important.