I think you have an Eastern Phoebe here. Was it wagging its tail up and down? The thing to remember about birds is that they are generally full size when they leave the nest, so even a juvenile Mockingbird that you might see would be the same size as an adult.
The things that distinguish young birds from adult birds (one they have fledged) are differences in plumage and other things like yellow in the corners of their beaks, rather than differences in size. It took me some time to learn this! Lovely shot..
p.s. I think what confuses people is that with ducks, swans, and geese you see that the offspring are smaller even after leaving the nest, so you think that it would be the same with songbirds, but it is not.
