Android app font size automatic setting

How can I set the text font size in the Android app?
My phone screen size is rather small compared to the 5" models and there is not much room for text in each line.

AFAIK you can’t set the font size in the App itself. You could try changing the default text size in the Android settings to “small”.

How can I contact the app creator to have this fixed ?
(font size should be proportional to the screen size/resolution).