how to decrease homepage size

Please try asking for assistance in the theme’s dedicated forum: https://www.ads-software.com/support/theme/ryu

In the meantime, do not edit the theme itself. First create a child theme for your changes. Or install a custom CSS plugin.

You can use a Child Theme to make modifications to your theme.

Add this to your functions.php file:

function custom_image_sizes() {
	add_image_size('home-images', 768, 300, true);
}
add_action( 'after_setup_theme', 'custom_image_sizes' );

You can then make a duplicate copy of the themes’ content.php file and place it in your Child Theme folder, make the following code change to it.

FIND:

<a href="<?php the_permalink(); ?>" title="<?php echo esc_attr( sprintf( __( 'Permalink to %s', 'ryu' ), the_title_attribute( 'echo=0' ) ) ); ?>" rel="<?php the_ID(); ?>" class="ryu-featured-thumbnail">
    <?php the_post_thumbnail( 'ryu-featured-thumbnail' ); ?>
</a>

REPLACE IT WITH:

<a href="<?php the_permalink(); ?>" title="<?php echo esc_attr( sprintf( __( 'Permalink to %s', 'ryu' ), the_title_attribute( 'echo=0' ) ) ); ?>" rel="<?php the_ID(); ?>" class="ryu-featured-thumbnail">
    <?php if (!is_home()) { ?>
        <?php the_post_thumbnail( 'ryu-featured-thumbnail' ); ?>
    <?php } else { ?>
        <?php the_post_thumbnail( 'home-images' ); ?>
    <?php } ?>
</a>

You’ll also want to Regenerate Your Thumbnails to ensure the new image size is being used on the homepage.

Hope this helps.