Just so you're aware: To visibly see the resolution that 4K offers, you must be sitting closer than 1.5x the screen width. So, with eyes to screen at 9' away, you would need a TV 6' wide just to visibly see a resolution jump from 1080p to 4K.
Movie theaters tend to put the center seats at about 1.5x screen width. Many people like sitting a bit closer than 1.5x screen width.
So, using that, your screen would be very comfortable at 72" wide, which is 83" diagonal.
At 10', a 65"+ television is not at all uncommon and something most people get used to in a couple of hours. Yes, I know that sounds crazy.
Maximize the size which fits into your space. A shame you don't have that extra half inch to get a 55" display, but definitely go for the absolute largest which fits your available space.