I don't think there is a right size. Just go for whichever you like. In truth reading OP's posts, it seems like he wants to go for the 55''OLED so go for it. If you go to a TV store and check out a 65'' and 55'' TV , you'll know straight away if you want to go bigger or not. If the size doesn't affect you, get an OLED.
If I'd had a 75'' Panasonic 902B (they didn't exist so I had 65''), I would not have upgraded to my current TV. Despite the numerous technological advances, I think once you get to a certain level of PQ, what will drive you to upgrade is size (whether its because you get a bigger house, make a home theatre or want more immersion). I find upgrading for size is a more viable mechanism for forcing me to switch TVs than an upgrade in PQ.
If I'd had a 75'' Panasonic 902B (they didn't exist so I had 65''), I would not have upgraded to my current TV. Despite the numerous technological advances, I think once you get to a certain level of PQ, what will drive you to upgrade is size (whether its because you get a bigger house, make a home theatre or want more immersion). I find upgrading for size is a more viable mechanism for forcing me to switch TVs than an upgrade in PQ.