How to change button text size from theme?

I have an theme and button theme

..
ThemeData(
    buttonTheme: ButtonThemeData(
        textTheme: 

now I want to change text size but I understond I only can change text color from theme.

I can change it with textTheme in ThemeData

ThemeData(
    themeData: 
        TextTheme(
            headline6: TextStyle(
                fontSize: 12,
                fontWeight: FontWeight.w400,
                color: Color(0xffFFFFFF)),
)

but if I don't use onPressed property, textColor is not changing. It's white too.