We've done a bit of testing of font size options in Android and iOS in regards to how well they cascade down to the Moodle mobile app.
It seems Android is quite good Display > Font Size & Display Size do have an effect on the app. The iOS version of the app doesn't seem to support dynamic type which is required to take advantage of iOS font size changes.
It would be great if students could customise their view a little further than font size. I'm thinking of students with Specific Learning Difficulties or Visual Impairments where changing foreground / background colours and font sizes could make a big difference to the usability of the app for them.
As a basic solution if a student could select from a list of preset style sheets under settings (specified by an administrator under Plugins > Web services > Mobile) eg. "Green bg, Yellow fg, 130%", "Cream bg, Dk Blue fg, 125%" etc. This would be a good starting point (obviously not ideal but maybe a good place to start).
Ultimately it would be great to have an Aa icon right next door to the search icon which introduced functionality similar to the excellent Moodle accessibility block.
I'd be very interested in helping out on the development or testing of this kind of functionality.
All the best.