Always get the bigger TV, always! Every time you watch it, you will be asking yourself if you should have got the bigger one. You are totally overthinking this.I would have a bigger one, but my better half has a thing about the TV hanging over the entertainment center, don't want to buy a bigger one and a bigger TV.
65 is really not that big, vs a 55, it's less than 10in bigger the way they are measured. Think my bedroom TV is a 55in.
My room is slightly bigger, sit about 12ft back, have a 9.4.2 system, all horn speakers. My TV is a 85in, still thinking about getting a 95-100in. For movies, the center is the most important speaker, think something like 80% of all movie sound comes from the center speaker. Getting all the levels correct is very important. The entire front soundstage should be the same level across the board, where you can't tell if the sound is coming from the center or the sides, unless it's panning, if so it should be seamless.
The Klipsch R34 is a good little center, easy to place, and not very big. But it is prone to reflections, if you place it on anything, it should be at the edge, so no driver is reflecting off the top of bottom of the cabinet/shelf. It's also good to have all your HT speakers timber matches, as sounds pan/move they all sound the same.