If you are planning for a tv, then yes, you would be viewing a 16:9 image the majority of the time. When CinemaScope movies come on that are filmed with a 2.35:1 aspect ratio, you will experience the black bars above and below the image.
That being said, here is a link for some general THX recommendations for tv. They recommend a maximum horizontal viewing angle of 40 degrees. This means that from 10' away, you could go as large as a 100" diagonal tv without exceeding that angle. So no, a 55" or 70" tv will not be too big from 10' away. However, since viewing distances are subjective, you need to determine whether a 40 degree viewing angle is even comfortable for you. Experiment with different viewing distances in front of your current tv until you come up with a comfortable distance. Determine that viewing angle and use it to calculate the size of your new tv.
The front row in my room is very similar to your proposed seating distance. I sit 9'-6" away from a 125" diagonal 2.35:1 screen, which creates a 54 degree viewing angle. When I project a 16:9 image on the same screen (with black bars along the sides of the image), the image measures 100" diagonal. This gives me a 42 degree viewing angle (slightly more than the THX recommendation).