Go to
Sherrill Arborist Supply and search for "Stable Braid".
I bought a 150' 1/2" rope from them for $112.50 + shipping.
This rope is made by
Samson Rope Stable Braid The average strength for 1/2" is 10,400 pounds.
Samson makes most of the ropes used by arborists.
I use this rope and my 8,000 pound winch on my Jeep.
There are 2 things to watch for when using rope. The most important is to avoid "shock loading", that is jerking on the rope or otherwise subjecting it to an instant load.
The other thing is to watch how sharply you bend the rope.
There is some good information on the Samson site about using rope and taking care of it.
Ski rope, twisted nylon rope, and just about any other rope you find in a hardware store is NOT appropriate for tree work and can be hazardous to your safety.
I don't know what you mean by "pretty large" but where I live that would mean a pine tree 100' or so tall. Let one of those get away from you and you will have a real serious problem.
I beg you, if you are in doubt at all get a professional to take them down. I spent $1,000 2 weeks ago getting 7 trees taken down. But one slip would probably have cost $25,000 to repair my house. Be sure you ask to see the arborist's liability insurance policy before you let them start.
Bill Tolle