I’m sure there is a better way to do this other than overriding the plugin’s core files, however, since this plugin has not been updated in over 2 years, I’d say it’s safe to go ahead and change things without risking any major breakage.
Navigate to Plugins > Buddy Notification Bell > includes > class-notification-bell-public and CTRL + F and search for the following line $menu_title = '<div class="bnb-pending-notifications ' . $alert_class . '"><i class="fa fa-bell-o fa-2x"></i><span ' . $hide_count . '>' . number_format_i18n( $count ) . '</span></div>';
The line you are looking to edit is <i class="fa fa-bell-o fa-2x">.
The bell icon is currently enlarged 2x as you can see by the fa-2x. Check out FontAwesome’s documentation of icon sizing to change it to your liking. FONT AWESOME SIZING
