there's no max. Mine seem capable of climbing large tree trunks. But they prefer stuff they can grip more easily, perhaps the circumference of a Canadian dollar coin (a bit bigger than a USA or Canadian quarter) would make for a good "large" size, along with the smaller stuff
I have a piece of drift wood I found on the beach. I baked it and cleaned it a bit. It is around 3 inches wide and an inch in length. It has a lot of grooves and my cham loves to hid behind it.
I feel like once you put all the different branches in your cham will show you which ones are best. You will notice your cham have almost a paper route and traverse the same way through his cage on certain branches when ur cham gets acclimated to the cage.