Unless they make foldable screens that can serve as a phone when folded and as a tablet when unfolded, 5.5 is probably the biggest it will get.
I could go for a 6" + and a 5" standard model in a few years.
No way. Bigger does not mean better. There are absolutely people who'd love a 6" phone but there are most certainly people who want something smaller than 5". There's no way they'd make the smallest phone on the market 5". Just my $.02... but seriously.
3.5" felt satisfactory for a while, and customers will tire of a minuscule 5.5" eventually too.