Given that it is a mini key. Is it possible to use 256 bits ?
No, because the newer standard is still only 30 base58 characters and the first character is always "
S";
Even if you managed to device a way to use a complete 256bit entropy to generate it,
the security wont still be 2^256 since an attacker could just go for the mini private key itself, and not the entropy.
It was a site walletgenerator.net
Oh, so it's not entirely related to mini private keys but about bad entropy.