WHAT constitution written and ratified by WHOM? If you don't think massive authoritarian corporate interests are the ones writing the rules you're beyond ignorant.
A global constitution written by the the nations leaders representatives, or more likely a community, and then voted on by those reps, needing a high majority to be accepted.
Wouldn't be up to me, and depends on if they have Senstors of a kind as well. I am guessing they would go with each nation getting one. Representing regions makes more sense.