the problem is that people tend to have different preferences about phone size so you cant please everyone with only 1 model.

my problem is that its not a matter of preference, id love to have a huge screen, but i cant.
when on the go i use my phone in portrait, and a 5" phone (16:9) is the largest that im able to use (with bezel, might be a little bigger for bezel-less phones) with one hand.
i dont want to go about dropping everything (but the phone) to do anything useful with it, if i cant type on the portrait onscreen keyboard with one hand it aint the decvice for me.

one thing i dont understand though

where do you find a design for a qwertyslider that big so as to not have to develop it from scratch?
the last landscape qwertysliders were produced way before screens got that big, so ... you'd asume there arent any designs that big you could use?

question: have you considered a touchscreen phone with optional magnetic keyboard? (like tohkbd https://www.youtube.com/watch?v=A834DyFiBUE)
you'd have a wider range of designs to choose from, just need to add pins and magnets on the back.