More often than not, I run across spaces where the rug is clearly too small. My rule of thumb is the larger the better. Typically, I take custom rugs a foot off of each wall, but when dealing with ready-made rugs you can still use this as a guide. As long as you are defining the space appropriately with all of the furniture on the rug, it will work. It is amazing how a proportional rug grounds the space and warms it up.
If you need help pulling together your space, call Lauren Andrews Design at 404-989-2386.