I used a 32" non-curved 4k monitor for a few months once. At some point I realized that I was moving my head around a lot as the corners were at an awkward place. On 28" I don't have this.

So anything above 30-ish inches I would consider either curved (expensive for hidpi resolutions) or two/three 27" screens angled a bit.

I can't imagine how bad it would be on a 65" flat screen.