Basically, ~7cm width is about the limit it seems, because I assume manufacturers realised that's where most human hands are comfortable with holding a phone.
There were a few giant phones in the past, Sony Xperia's Ultra at 6.4 and the Xiaomi's Mi Max at 6.44" to 6.9". If given the 19:9 to 20:9 aspect ratios of today they'd be 7" phones (so again, don't compare phone sizes based solely on screen diagonal). As they're no longer made I assume they just didn't sell well enough.