It depends. What do you want the decorative carpet for?

If you plan on putting it under a coffee table, then 5x8 is a good size.

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If you plan on a conversation area of sofa/chairs/tables in front of a fireplace or tv, then I would draw out to scale the measurements of the area of the room where this will be and measurements of the furniture and placement within that area. Then you will hava a good idea of the size of the rug you need. 5x8 might be a bit small for an area like that in a large room.

Our Gentleman's Parlor is 18 by 18 with a walkway along one length. We have a nine by eleven area rug. I can't imagine having anything smaller.
I agree with Patty to draw up a scale floor plan with the furniture placement before you buy a rug if you really want to buy a rug now. Personally from experience of building three homes I'd wait until you were in the home. You won't really have a feel for what works until you are in the home.

Purchase an inexpensive rug pad. Arrange furniture on the rug pad. This should give you an idea of the size of rug you would need in the sitting area.
Just a thought.