Font size adjustments are possible in several places.
In the account list: Open the top right menu > View
In any folder: Open the top right menu > View
In an e-mail or compose screen: Swipe up the bottom menu > Tap More > Use the A+/A- options

Adjusting the font is also possible with your phone or tablet's Volume Up / Down keys within the same places. Note that these controls via volume buttons only apply for phones and tablets and do not work on devices using a physical keyboard (QWERTY, QWERTZ or similar).

If using a tablet or two-panel mode, the Volume Up / Down keys affect the right window, while the menu option affects the left window.