Good idea. But maybe breaks compatibility with the current behavior of setPasswort() for existing installations of phpseclib.
At least, $password_key_size is anyway just a default value. With the 7th arg of setPasswort() each user can define dkLen self, if needed. 

Except AES/Rijndael.php, currently, all ciphers has set $password_key_size to the highes possible value.

Maybe we should discuss this issue when setPassword() will support PBKDF1 or similar low-kdf ... for a solution.

However... i let it by you then Patrick... the default value $password_key_size ... 
Personaly i think a 64bit key as default for setPassword() is much too low/insecure for PBKDF2 kd. 

Would suggest at least 128bits... $password_key_size = 16