Changing size of user avatar in template file
This is definitely the command you want to run

theme('imagecache', 'your_custom_preset', $picture, $alt, $alt);


You will have to install imagecache module for Drupal 6.  Its in core for Drupal 7.   You have to define 'your_custom_preset', in the imagecache profiles.

The other place you can theme the avatar is inside the function
[YOUR_THEME_NAME]_preprocess_user_picture(&amp;$variables)  and again use the same line as above. for more information checkout the api.drupal.org documention on this issue