A user has alerted me to this issue. Selecting a different text size in the personal settings area does not seem to modify the display of text. I’ve tried several of the standard themes. I’ve saved changes at each modification and reloaded pages. I don’t see smaller or larger text.
Is anyone else experiencing this problem?
I’ve just adjusted mine here on Meta, and as soon as I select a different size the text updates as a preview, and sticks when I save (I’m currently on Android/Chrome).
Can you replicate it here on your device?
(I’ll add a link here to https://meta.discourse.org/my/preferences/interface as I tested this on mobile and couldn’t use the notification menu to easily get back to the page. 
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I’m seeing that here on meta, which is good, it means that it should work.
I’m on a hosted instance of discourse, so I’m expecting to get out-of-the-box behaviour using standard themes and our users are griping because they can’t adjust the display of text to suit their eyes.