See the following for more information:

Font Size

The "Font Size" setting controls the font size level you wish to see when reading eBooks. The ePub Reader provides seven levels of font size for use, namely "XX-Small", "X-Small", "Small", "Medium", "Large", "X-Large" and "XX-Large". The actual size associated with each font size level can be set through the Font Sizes Settings.

The default font size is "Medium".

Default Display Settings
1. Default Display Settings
Set Font Size
2. Set Font Size
Select Font Size Option
3. Select Font Size Option
Font Size Option Applied
4. Font Size Option Applied