There's a bug in mod_k2_users v2.8.0 which causes it to ignore any custom avatar width set in the module configuration. The bug is helper.php line 180.
Erroneous code:
$userObject->avatar = K2HelperUtilities::getAvatar($userObject->UID, $userObject->email, $params->get('userImageWidth'));
Corrected code:
$userObject->avatar = K2HelperUtilities::getAvatar($userObject->UID, $userObject->email, $params->get('userAvatarWidth'));
Please verify and include the fix in the next release. Thanks.