Like extra said, at least half inch. I use a 150' around 8,000 pounds strength, and a 160' five eigths, I think twenty thousand. Stable braid. For back up I keep a 100' one inch line also. I think they're all Samson. Color code with a plan, I use orange or blue for climbing, white or red for pulling and lowering. extremetree has a good idea with the 9 sixteenths, I may try that next time, the five eights is clumsy for some groundies to handle. If I ever need an inch and a half I'll probably refer the job to someone else.