How to change the Font Size in a whole Application programmatically, Android?

Android documentation is not specific on the most efficient way to change the font size globally through the user’s selection at application level.

There is a problem I think with the answer given by Black Devil.

The problem is that many of the Android widgets subclass TextView, such as Button, RadioButton, and CheckBox. Some of these are indirect subclasses of TextView, which makes implementing customized version of TextView in these classes very difficult.

However as pointed out by Siddharth Lele in his comment, using styles or themes is much better way to handle change in the text size throughout the app.

We set styles for layouts to control the look and feel of the view. Themes are essentially just collections of these styles. However, we can use a theme just for text size settings; without defining values for every property. Using a theme over styles provides us with one huge advantage: we can set a theme for the entire view programmatically.

theme.xml

<resources>
    <style name="FontSizeSmall">
        <item name="android:textSize">12sp</item>
    </style>
    <style name="FontSizeMedium">
        <item name="android:textSize">16sp</item>
    </style>
    <style name="FontSizeLarge">
        <item name="android:textSize">20sp</item>
    </style>
</resources>

Create a class to handle loading our preferences:

public class BaseActivity extends Activity {
    @Override
    public void onStart() {
        super.onStart();

        // Enclose everything in a try block so we can just
        // use the default view if anything goes wrong.
        try {
            // Get the font size value from SharedPreferences.
            SharedPreferences settings =
                getSharedPreferences("com.example.YourAppPackage", Context.MODE_PRIVATE);

            // Get the font size option.  We use "FONT_SIZE" as the key.
            // Make sure to use this key when you set the value in SharedPreferences.
            // We specify "Medium" as the default value, if it does not exist.
            String fontSizePref = settings.getString("FONT_SIZE", "Medium");

            // Select the proper theme ID.
            // These will correspond to your theme names as defined in themes.xml.
            int themeID = R.style.FontSizeMedium;
            if (fontSizePref == "Small") {
                themeID = R.style.FontSizeSmall;
            }
            else if (fontSizePref == "Large") {
                themeID = R.style.FontSizeLarge;
            }

            // Set the theme for the activity.
            setTheme(themeID);
        }
        catch (Exception ex) {
            ex.printStackTrace();
        }
    }

Finally, create activities by extending BaseActivity, like this:

public class AppActivity extends BaseActivity{
}

As most of the application have a much fewer amount of Activities than TextViews or widgets that inherit TextView. This will be exponentially so as complexity increases, so this solution requires less changes in code.

Thanks to Ray Kuhnell