Add the following code snippet to your theme’s functions.php file to modify the size of the author box Gravatar size.

// Modify the author box Gravatar size
function author_box_gravatar_size( $size ) {
    return '80';
add_filter( 'genesis_author_box_gravatar_size', 'author_box_gravatar_size' );

Currently, the Gravatar size is set at 80px. To change the Gravatar size, modify the size in pixels on line three (3) of the above code snippet.