The play with the measure quite a bit.
I think it's said to be 3 or 4 inches on each side in book 1, but in 50 Jake pulls it out of his pocket. I know I couldn't fit a 3 inch box in any of my pockets
A red-tail is probably around 2 feet long, with a 3-4 ft wing span, so it shouldn't be more than 1/5-1/4 his height I think. Ospery would be similar in size. I have no idea what the size of his talons would be, probably smaller than an adult palm.
Maybe that helps, but think you'd be better off just finding a size that's aesthetically pleasing. Beauty of books, in the end you decide what it looks like