most super admins have the same space allocated across the board.
I *think* there might have been a plugin somewhere to show this, but it;s not built in, unless you want to visit the dashboard of each site.
I use something like the following as part of an mu-plugin that displays a variety of stats per blog on the usual list of blogs at SuperAdmin->Sites
<?php
function ds_upload_space_used() {
global $blog;
$dirName = ABSPATH . 'wp-content/blogs.dir/' . $blog[ 'blog_id' ] . '/';
echo '<strong>Space Used: '. ( round(get_dirsize($dirName) / 1024 / 1024, 2)) . ' MB</strong>';
}
add_action( 'wpmublogsaction', 'ds_upload_space_used');
?>