i use cpanelextend module, open up the module inside components folder and find the main php files, here for cpanelextended the files is; cpanelextended.php, then find; function generatePassword , and look for ;
public function generatePassword($length = 8) {
replace to 12 and it would be like this;
public function generatePassword($length = 12) {
then finish..
see here for more details;