As far as I can tell I've done exactly what you are sugesting.
I've modified 2 files. See below:
In the file - jcomments.php I added the size=128 attribute.
if (empty($comment->avatar)) {
$comment->avatar = '<img src="
http://www.gravatar.com/avatar.php?gravatar_id='. $comment->gravatar .'&size=128;&default=' . urlencode(JCommentsFactory::getLink('noavatar')) . '" alt="" />';
In the file - style.css I modified the width attribute
/* Avatar */
#comments .comment-avatar img {width: 128px;border: none !important;}
With the modified CSS I get a larger avatar/gravatar, but it's blurry.
Am I doing something wrong?
Thanks so much for your help.