The room might might not be set up for a rug, too...we just don't know.

As far as size I would look up rugs that are 7x9 ft. Then I think the rug should sit just under the sofa and the wing back chair and still leave some of the wood floor showing.

I agree with Mark's comment re: size and placement. As far as color, it would be good if it included the taupe of the sofa and the blue & green of the chair.

I was thinking this rug from West Elm:

Does anybody have any thoughts on re-arranging the room? Perhaps moving the couch in front of the window, the coffee table in front of that and then have the couch and wingback chair facing each other. Then we could get a 6 by 9 rug that would run from underneath the couch to underneath the wingback chair. Where the couch is now, we could put some small wicker chairs facing the secretary.

It's a little difficult to know all dimensions from photos; e.g. is there room width-wise to rotate the sofa 90 degrees with the front of it lined up with the right wall where it juts back out? If so, you could then put the chair in the corner to the right of the window and potentially put a long bookcase in the cut-out under the pictures.

I think that your secretary is creating the problem. It is blocking a clear pathway from your front door. If you could remove it to another location, that would help to make your sitting room the focus. I would measure the room starting at the inside of the entryway to the sitting room (about four inches into the room from where your measuring tape is in the picture) to the wall behind the sofa as your width. The length of the room is from your window wall to the jog in the wall to the right of your sofa. Once you have those measurements subtract approximately 18" all around to allow for a border to the rug. The resulting measurement should give you a good idea of the appropriate rug size.