WordPress has default size for post author avatar. If you increase or decrease the size, use following code.
Go to place that you want to display the avatar.
$author_avatar_size = apply_filters('hs_author_avatar_size', 100);
echo get_avatar(get_the_author_meta('user_email'), $author_avatar_size);
Above code tells WordPress that, display author avatar image with 100px width and height.