In the latest release they didn't add a paper size we use around here pretty often if we're going to print the dashboard out for mock ups, editing, shipping to people. Tabloid (11x17) . Tableau uses 100 pixels per inch on their dashboard. Making the math easy to figure out the dimension: 1100x1700.
I limit my use to landscape as I haven't found a way yet to zoom out from the dashboard screen. I guess I could always turn my monitor sideways :)