*Shudders*
I think we're arguably being a bit closed minded when we imagine a standard form-factor display and thus try to reverse-engineer a size from the 8K rumour.
Looking at what displays are hitting the market recently, curved displays are emerging, for example the LG 38UC99 (and others in the series and from other manufacturers). Curved displays seem (understandably) to have died in the TV sector, but they make far more sense in the desktop setting. Indeed, the present paradigm of multi-display setups is a product of technological limitation rather than design intent. I can envision, for example, an Apple display in 2019, alongside the Mac Pro, that is curved and ultra wide - and thus technically 8K, but not in the way we are thinking about it.