How to add Custom width & Height
to our custom image uploader

function clientPic6( $wp_customize ) {   
    $wp_customize->add_setting( 'client_background_img_6'); // Add setting for logo uploader
    // Add control for logo uploader (actual uploader)
    $wp_customize->add_control( new WP_Customize_Image_Control( $wp_customize, 'm1_client_img_6', array(
        'label'    => __( 'Client Image', 'm2_C6' ),
        'section'  => 'custom_client_block',
        'settings' => 'client_background_img_6',
    ) ) );
}
add_action( 'customize_register', 'clientPic6' );

Read more here: How to add width & height (resolution) on wordpress function?