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Two is better than one. Less weight to lower and raise, less area to lose heat, and no loss in functionality (unless you're wanting to stuff a whole hog or something else "big" on there. Dimensions are up to you--what you have marked is probably about what I'd do--maybe just a bit smaller. That's all personal preference/trial and error.
I'd say at least 4"--6" if you can get it. It's easier to close off a pipe that is too big (using your damper) than it is to open one up farther when it is too small.