Custom snippet that will disable site size calculation. In case of inability to connect a child site, it is good to try to use this on the child site.

disable-get-site-size.php

<?php

// Add the following code snippet to the functions.php file of the active theme of your Child Site site.

add_filter( 'mainwp-child-get-total-size', 'mycustom_mainwp_child_get_total_size', 10 , 1 );
function mycustom_mainwp_child_get_total_size( $value ) {
  return false;
}

?>