Change your font size with the following steps:
Head to "Settings" and scroll down to the "Quran Appearance" section
Tap on "Mushaf Layout", then select "Quran Text" as your Reading Layout.
You can change the font size of the Arabic Quranic Text for when you use Tarteel in the "Quran Text" layout mode.
If you want to change the font size of the Translation and Transliteration, select "Translation / Transliteration" and modify the font size as desired.