It has been a while, but figured I would post an answer seeing as others may have the same issue.
You are correct, the URL for your devault gravatar goes after the 40
Example before and after:
Before
<?php echo get_avatar( $comment, $size='60' ); ?>
After
<?php echo get_avatar( $comment, $size='60', 'https://insert-your.default-gravatar.replacement-image-url/here.png' ); ?>