I think 7 - 8 feet in length would look proportional if your total sofa back length is 123 inches. I usually shoot for about 2/3 of the length of sofa for a statement piece of art.

try masking the size out on your wall with painter's tape and see what size looks best to you in the room.

Thanks to both. The room is just now being sheetrocked and sectional won’t be here until July. I like the 2/3 rule of thumb and even if I can’t get the full effect, I will mask it out this weekend when the workers are gone. We are very much social distancing through this project. We don’t go down during the week, and trades come and go through the basement sliders.

“try masking the size out on your wall with painter's tape and see what size looks best to you in the room.”

This is exactly what I would do also.