Pick a book/article you want to read and on the top right, tap the three dots button. This should open up several options for you. Tap on “Change Appearance”.

The second and third row, lets you change the font and font size that you want to use for your reading pane.

Tapping the font name gives you 8 different options which you can choose from.

For the Font size, tap the '+' to make text appear larger, or '-' sign to make the text smaller