Late Roman Unit Sizes

[Renatus]

Aren't you confusing the Mattiaci with the Mattiari?

There are units of Mattiaci and units of Mattiarii in the Notitia, so they are clearly not the same. Bringing the tribe into the discussion just confuses the issue. The suffix -arius often means something like 'equipped with', e.g., sagittarius (pl. sagittarii), which is no doubt why it has been suggested that the Mattiarii got their name from a weapon called mattium but what it was, whether it was a weapon at all or whether the etymology is entirely different remains a mystery.