1. You don’t have to change the code. You can add the following line to your wp-config.php file (and change the value) to change the avatar size:
define( 'FOOTBALLPOOL_MEDIUM_AVATAR', 28 ); // size in px
2. The user predictions (I guess you’re referring to the overview of user predictions for a match) does not have a template (yet). So you will have to alter the plugin’s code for that. Remember that when you change the code, you won’t be able to upgrade (at least not without losing your changes).
The function used for the display of match predictions is:
Look for the line that shows the user name and change it to:
$pool->get_avatar( $row['user_id'], 'medium' ) . $pool->user_name( $row['user_id'] )
Or, you can use the footballpool_statistics_matchpredictions_html
filter to alter the HTML for that page. There is an example in the help file that changes HTML with the help of the Simple DOM ‘plugin’; you can try using that technique, but it requires some more knowledge of PHP.