How would I write my initials if I have two first names?

Example:

Maximus Decimus Meridius Exodus

Maximus and Decimus are my first names. Meridius is my mother’s surname. Exodus is my last name.

Do I just leave out Decimus and just do M M E? Or do I do M D M E? Four letter initials or 3 letter initials?

How does it usually work?