How big is safe

How big does a limb need to be before you feel comfortable using it for life support?

It depends on the type of tree, the angle of the branch and whether a branch/trunk union has included bark. Also the weight of the climber. For example I weigh around 150lbs. With most white oaks I feel comfortable on branch crotches with a minimum of 3" to 4". I would never climb on a Tulip tree branch that small.

When ever possible use a friction saver around the trunk instead of the branch.

I usually try to avoid branches with tight crotches and included bark. Included bark is bark that gets trapped between a branch and the trunk as the tree grows. This is usually visible by a bark ridge that extends from the crotch in toward the trunk. Most branches have a small bark ridge however if it is more than 1/3 the branches width I would be hesitant to rely on the branch alone.

This kinda requires a multi part answer.

Generally speaking, for most hardwoods, to hang my rope from the branch, the branch must be at least as big as my arm where it connects at the shoulder. And even then, I'll only go out from the trunk about 2'. Tops.

Of course, the bigger the diameter of the branch, the farther out from the trunk I'll hang my rope,...IF I KNOW THE TREE. Can't stress that part enough.

The 1st time I climb a tree, I always run my rope around the trunk, so I can inspect it on the way up. Only after I'm really confident in the soundness of a particular trees structure will I "go out on a limb". Sorry, I couldn't help myself on that one.

Now, If my rope is going around the trunk, I'll allow the limb that's holding my rope in place on the trunk to be about as small as my thumb. NOTE...I was a working arborist for a number of years, and I know from a lot of experience that this works for me. I'm not telling you to do this. I'm just sayin. You do with that info what you will. YOU ARE RESPONSIBLE FOR YOU.

For the record, I go around 190#'s