ISO has not issued standards for naming of humans (yet), though I hear a preliminary draft is being passed around at the very highest levels.
:)
Seriously, 64 or so should be adequate for 99.99% of first and last names -- in any language. I've never seen anything other than one char for an MI (though you're right, many folks have more than one).
BTW: Unless I'm mistaken, email addresses -- technically -- can be up to 320 characters in length and can contain ! # $ % & ' * + - / = ? ^ _ ` { | } ~ and . provided that . is not the first nor last character, nor may it appear two or more times consecutively.
PS. My pet rabbit's name is "His Imperial Majesty Theopolops Bunny Galore". Needless to say this was truncated in the vet's database. I think they have it as "His Imperial Maj". So it goes.