Try a TV WITH speakers or at least a soundbar - the difference is phenomenal. You pay decent money for a TV then use their rubbish inbuilts? Yuk!
As for size, 65" is plenty big enough for that room - money you have saved, get some decent sound for the full immersive experience.
Hey, I did say above that we are getting a Sonos Arc soundbar. I don't have the space really or layout/suitable room for surround and don't really want that.
I won't be using the in built audio, all will go through the Sonos Arc