The phone that I use right now (Moto G Stylus) is about 15cm wide when I use it in landscape mode. It is perfectly usable that way, except that it is nowhere near tall enough.
Giant phones like 6.9" are not from the past, they are also in the present, like the “pro” versions of the Pixel and Iphone. Why are they almost 7" instead of actually reaching 7"? That’s what I’m trying to understand. I’m very skeptical of the “they didn’t sell well enough” theory, which sounds to me like system justification. It also doesn’t explain why I can’t buy a phone sim and put it into a tablet. So absent concrete evidence, it still sounds to me like carriers are getting in the way of larger phones. I do see though, that the Samsung Z Fold3 foldable has 7.6" diagonal when unfolded, so maybe that refutes the carrier theory.
I think people might be less concerned these days than before about pocketing or handholding the phone, since they use connected wearables like the Apple or Google watch for small screen functions like phone calls.