Unless otherwise indicated, the code snippets you see below should be placed into your theme’s functions.php file.
Here is the code needed to modify the size of the Gravatar on your site:
<?php
//* Do NOT include the opening php tag shown above. Copy the code shown below.
//* Modify the size of the Gravatar in comments
add_filter( 'genesis_comment_list_args', 'sp_comments_gravatar' );
function sp_comments_gravatar( $args ) {
$args['avatar_size'] = 96;
return $args;
}