What would be the standard vexillation size?

[Frank]

The eagle standard           1st Vexillation
The wolf standard.       ....2nd Vexillation
The minotuar standard.....3rd Vexillation
The horse standard...     ..4th Vexillation
The wild boar standard.....5th Vexillation

Surprising input Antiochus. You are calling the roots of the term vexillatio into play.

So you are saying, that the mid-republican legion (and earlier) was not just organized in 30 manipuli, but also in 5 vexillationes according to the the 5 old standards? This would make sense, if every manipulus also belonged to a certain standard. After the marian reforms manipuli and the old standards became meaningless. Nevertheless, a lot of the old terms survived. Romans have been very traditional, especially if it comes to legions. So afterwards "vexillatio" stayed to be the term for 1/5 of a legion, which were now 2 cohorts. 

On the other side, vexillatio became also a general term. Every detachment from 20 to 2000 men could be a vexillatio. But the traditional size was still 1/5th of a legion. Until the former vexillationes became legiones comitatenses of 1000 men, and the term vexillatio was now used for a cavalry unit in late empire.

Interesting approach. I am just not sure, that we got enough evidence for your theory.