Sled ropes

Its been years since I've pulled a sled into the woods but am planning a snowshoe trip for after Christmas with my grandson, his first ever winter camping trip, and I'd like some suggestions on recommended rope lengths over pond and generally flat terrain.
I'm thinking 70 to 80 inches but have read 100 or more which seems darn long to me, and down to 40.
Anyone here with experience have their suggestion?