I have not seen any dimensions most people free-form them. Here are some pictures of mine. The entire assembly attaches with one bolt through the stake pocket hole on each upright. The boards attach to threaded inserts so no hardware shows on the inside. I can measure them if you want. Also I did mine in oak which has remained very stable I am unsure about how the pine works.

http://www.flickr.com/photos/advance-design_parts_co/8129076158/in/set-72157631867396207