Is there a weight limit for zip lining
Yes, there is, but it varies by company so you need to check with your individual provider. Some do it by weight, and some do it by if you can safely wear the harness. The weight limits are pretty big though. For Sky Trek, I think it's close to 300 pounds.
The best and only safest way is to go to tour location...do not trust anything else....
Depending on which zip lining you are considering, and which option of activities within one attraction in some cases, you can check out the FAQs and/or Restrictions section on their site. E.g., https://skyadventures.travel/skytrek/ shows 300 lbs for Sky Trek, but 216 lb max for a Sky Limit circuit, which includes zip lining (https://skyadventures.travel/skylimit/), etc.