You need to hook into wp_admin_bar
action and alter the corresponding object. Something along these lines will do the trick for you. Place the following code to your functions.php
.
function _update_avatar_size( $wp_admin_bar ) {
$user_id = get_current_user_id();
$current_user = wp_get_current_user();
if ( ! $user_id )
return;
$avatar = get_avatar( $user_id, 30 );
$howdy = sprintf( __('Howdy, %1$s'), $current_user->display_name );
$account_node = $wp_admin_bar->get_node( 'my-account' );
$title = $howdy . $avatar;
$wp_admin_bar->add_node( array(
'id' => 'my-account',
'title' => $title
) );
}
add_action( 'admin_bar_menu', '_update_avatar_size', 999 );
(Don’t forget to override the admin stylesheet to display the larger avatar.)