The one I made basically followed HE's directions for baffle size (~7.8") and mine was 3" thick. With some overfill the baffles will expand thicker, therefore causing them to be shorter (probably something like 4-5" thick which will shorten the baffle and inch or two). IMO, a 15" X 3" baffle with overfill could expand 6" or 8" therefore causing the baffle (and overall length) to shorten by several inches. Of course this would be with the quilt "at rest" so to speak. Using it you would probably pull it back out to full length. But like above, I believe 15" may be a little wide and could cause shifting easier.
If your concern is using 1 bag per baffle to make it easier and not have to weigh the down you might consider halving the baffle and then just cut the bag of down in half to stuff each baffle. You could probably do this without weighing or measuring.
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