Create custom image sizes & increase font size

@sorayacge: To change the size of the featured image size used you will need create a child theme and do a little bit of customization. Let me know if you are familiar with child theme customization otherwise I suggest you to hire a customizer.

To increase font size please post in your site URL and let me know which text you want to be updated.

Kind Regards,
Skandha

@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

@sorayacge: Yes, the above code will only change featured image size.
Go to => Child Theme Folder => functions.php and the following Code.

function child_theme_setup() {
    add_image_size( 'slider', 1600, 600, true ); //Featured Post Slider Image
	add_image_size( 'featured', 800, 324, true ); //Featured Image
	add_image_size( 'small-featured', 400, 267, true ); //Small Featured Image
}
add_action( 'after_setup_theme', 'child_theme_setup', 11 );

You can change the images size of the above.
Let me know if this helps you out!
Kind Regards,
Skandha