It's in settings/display and brightness.

There's (I think they are new) controls for text size, which is a slider on it's own page and also a toggle for bold text. I have the bold text option enabled on my 6, it's much easier for my old eyes to read. I played with larger text size also, but didn't like that as much as just the bold text.

There's also a setting for "Display Zoom" on it's own page, you can choose "standard" or "zoomed", and it shows three different screens as examples of how they look in either mode, so you can compare. I tried that too, but for me, the best option was just bold text and standard mode on the 6.

Anyway if you get a chance, play with all those setting on the 6+ and then see what you think of it.

-David