The larger end of the cabers I've tossed (or tried to toss as a very amateur person and probably too old to start such) was about 4"-6".

At 2.54 cm /inch, that would work out to about 10.25 cm - 15.25cm.

That'll give you an idea of the size of the tree (or whatever) you need to secure. And be sure to check the weight since green wood is pretty heavy. If you cut some now and put them where they can dry out, you'll be better off.