I found this feather in Eastern Washington, specifically northern spokane. I estimate that it is roughly 20 cm long and 5-6 in width. Any help is much appreciated.
Not to pile on, but I also agree with wild turkey. In general, tail feathers (unlike wing feathers) have about equal amount of material on either side of the rachis. If it is a large feather with that balance (such as this one), then you start looking at grouse and turkeys.