Displays allowed and used space in GB or MB

wpmudev_dashboard_quota.php

<?php
/**
 * Plugin Name:       WPMUDEV Dashboard Quota
 * Plugin URI:        https://premium.wpmudev.org/
 * Description:       Displays allowed and used space in GB or MB
 * Version:           1.0.0
 * Author:            Panos Lyrakis @ WPMUDEV
 * Author URI:        https://premium.wpmudev.org/profile/panoskatws
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.html
 */

if( !class_exists( 'WPMUDEV_Dashboard_Quota' ) ){

    class WPMUDEV_Dashboard_Quota{ 
		
		private static $_instance = null;

		public static function get_instance() {
            if ( is_null( self::$_instance ) ) {
                self::$_instance = new WPMUDEV_Dashboard_Quota();
            }

            return self::$_instance;
        }

        private function __construct() {

        	add_filter( 'dashboard_glance_items', array( $this, 'dashboard_glance_items' ), 10 );
        	add_action( 'activity_box_end', array( $this, 'activity_box_end' ), 10 );

        	add_action( 'admin_head', array( $this, 'hide_default_quota' ), 10 );

        }

        public function dashboard_glance_items( $elements ){

        	$elements[] = self::display_quota();

		    return $elements;

        }


        public function activity_box_end(){

        	echo self::display_quota();

        }

        public static function display_quota(){

        	if ( !is_multisite() || !current_user_can( 'upload_files' ) || get_site_option( 'upload_space_check_disabled' ) )
	        return true;
	 
		    $quota = get_space_allowed();
		    $used = get_space_used();

		    $quota_str = self::format_space( $quota );
		    $used_str = self::format_space( $used );
		 
		    if ( $used > $quota )
		        $percentused = '100';
		    else
		        $percentused = ( $used / $quota ) * 100;
		    $used_class = ( $percentused >= 70 ) ? ' warning' : '';
		    $used = round( $used, 2 );
		    $percentused = number_format( $percentused );
		 	
		 	ob_start();
		    ?>
		    <h3 class="wpmudev_mu-storage"><?php _e( 'Storage Space' ); ?></h3>
		    <div class="wpmudev_mu-storage">
		    <ul>
		        <li class="storage-count">
		            <?php $text = sprintf(
		                /* translators: number of megabytes */
		                __( '%s Space Allowed' ),
		                $quota_str
		            );
		            printf(
		                '<a href="%1$s">%2$s <span class="screen-reader-text">(%3$s)</span></a>',
		                esc_url( admin_url( 'upload.php' ) ),
		                $text,
		                __( 'Manage Uploads' )
		            ); ?>
		        </li><li class="storage-count <?php echo $used_class; ?>">
		            <?php $text = sprintf(
		                /* translators: 1: number of megabytes, 2: percentage */
		                __( '%1$s (%2$s%%) Space Used' ),
		                $used_str,
		                $percentused
		            );
		            printf(
		                '<a href="%1$s" class="musublink">%2$s <span class="screen-reader-text">(%3$s)</span></a>',
		                esc_url( admin_url( 'upload.php' ) ),
		                $text,
		                __( 'Manage Uploads' )
		            ); ?>
		        </li>
		    </ul>
		    </div>
		    <?php

		    return ob_get_clean();
		    
		}

		public static function format_space( $space ){

			$format = __( ' MB' );

			if( $space > 1024 ){

				$space = $space / 1024;
				$format = __( ' GB' );

			}

			return number_format_i18n( $space, 2 ) . $format;
		}


		public function hide_default_quota(){

			$screen = get_current_screen();

			if( $screen->id != 'dashboard' ){
				return;
			}

			?>
			<style>
			.mu-storage{
				display: none;
			}
			</style>
			<?php

		}

    }

    add_action( 'plugins_loaded', function(){
		$GLOBALS['WPMUDEV_Dashboard_Quota'] = WPMUDEV_Dashboard_Quota::get_instance();
	}, 10 );

}

Not recommended, it's mostly for experimenting