bb-avatars gives you the possibility to change the size of the (gr)avatars shown. You can also choose what the standard (gr)avatar should be (for useres with no Gravatar). You can choose from Wavatar, Identicon and MonsterID. You can also use your own, by giving the plugin the URI to the picture you would like to use. You can also control the rating (X, PG, G or R) of the Gravatars.
Download the latest version from here