Does the size of the nest matter for zebra finches & to have them cage less?

So i have 2 zebra finches Female and male they seem to be getting along well and all but they are cage less finches. apparently they love the tree we have in our house (doesn't know what tree) and we provided them clean water, seed, and vegetable. as i started to see signs of the female gathering material to make a nest it tend to fail because it would just fall off the branches. so i went to get her a palm sized nest. is a palm sized nest big enough if she start to lay eggs? because two of my finches can fit the nest but idk if it'll be able to hold the eggs inside. can you give me some advice?