55". It's mostly the proper size for where it's in. But a 75" 4k would be quite something especially if I set it up for PC gaming.
My living room layout now is terrible, there's a fireplace that is in the way, as right now there is no way to put my couch directly in front of the TV. What I plan to do is remove the fireplace and make it a blank wall, then I can either put the couch there, OR, put the couch where the TV is now and put the TV on the new wall. If I did that, I could put a bigger TV as it would be a bigger wall. Just need to run extra cat5 and power but not a big deal given I'd be re-drywalling a whole section anyway.
I don't really watch TV a lot though, it actually took me several years after I moved in to even buy one. It's always cold in my living room and I have to crank the furnace to get it comfortable there so I don't tend to even spend much time. If I did something about the heat and layout I'd probably spend more time there and watch movies and stuff. It's a plan. Eventually.