Reply To: Create custom image sizes & increase font size

@sorayacge: So first you will need to create a child theme. For that you can use Generate Child Theme Plugin. Install and Activate the plugin. Go to => Dashboard => Generate Child Theme => Select Parent Theme as Adventurous Pro and fill the other fields and click on Generate. Then will create and active the child theme.

Now, Go to => Child Theme Folder => functions.php and add the following Code.

function child_theme_setup() {
    add_image_size('featured', 500, 324, true);
}
add_action( 'after_setup_theme', 'child_theme_setup', 11 );

You can alter the image size according to your choice.
This should change the featured image size on your site.
Kind Regards,
Skandha