I've got a 55 inch OLED sitting 9 feet away, and that works fine for me and my wife, but then I just upgraded from a 42 inch Panasonic plasma I've had for nine years or so
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
A nice thing about the LG OLED is that it has small bezels compared to my plasma, so the OLED is less physically imposing than a same size plasma would have been.
You could try make a paper mockup of a 65 inch OLED TV to get an impression of size when installed.