Its that 600ppi that did it.
You don't need more then 200 ppi for a good print That means the finest detail will be 200th of an inch. If you can see better than that with the naked eye Good luck to you. So What to do well a A$ sheep of paper is 2291 pixels wide by 1646 high or visa versa if land scape.

So you have 21 inches by 7 . something at 600ppi so lets re size with re sampling checked and smart size set Maintain original print size unchecked and lock aspect ratio checked . Now change the res to 200 ppi and enter awidth of 11" that's 2200 pixels the night will be changed to match. Then OK and go to print.