Avatars 80x80 are way to big for me. As there is no such a thing like "best size", it could be left for the user to choose.

For example:
patch bugnote_view_inc.php <<!
114c114
< <?php if ( ON == config_get("show_avatar") ) print_avatar( $t_bugnote->reporter_id ); ?>

<?php if ( ON == config_get("show_avatar") ) print_avatar( $t_bugnote->reporter_id, $g_avatar_size ); ?>
!

and then one might add a line to config_inc.php:
$g_avatar_size = 40;