I would refer you to the 1979 ruling - Burch v Louisiana. Here, the court established that a jury must be composed of at least 6 people and that a non-unanimous conviction by juries of less than 12 people is not valid.

I think you would find that the 6th amendment was written intentionally vague, because the original colonies were not in agreement as to the structure of their judicial system.

I do not know how far back 6 person juries go in the US, but it has been a while. Check this archive.org article from 2015: https://web.archive.org/web/20150602210951/https://www.uscourts.gov/services-forms/jury-service/types-juries

At the end of the day, I think you are right based upon the equal protection clause of the 14th Amendment. It was based largely on this, that Louisiana's non-unanimous jury convictions were overturned. I am torn from a perspective of state sovereignty.