Best practices when encrypting a file with three different ciphers
I) There is no minimum. Maximum in your case is $2^{64} * 16$ bytes with same key.

II) No. No. GnuPG takes care of that.

III) No. Compressing does not weaken encryption.

IV) I don't think it matters because it is not authenticated encryption.

V) Yes, passphrases must be different.

VI) In your case you can get maximally 256-bits of security per cipher. You should use around 50 characters (depending on complexity) per passphrase. More does not hurt.

VII) I don't think you can tweak anything. It is "best security" by default.


It is important to note that GnuPG does not authenticate files encrypted with symmetric encryption, which means you cannot detect, if it has been modified.

You can read more about cascade encryption here: https://blog.cryptographyengineering.com/2012/02/02/multiple-encryption/