What would be the standard vexillation size?

[Nathan Ross]

I have found other references to vexillations being termed a cohort, and it is a common practice among ancient historians.

If the vexillation consisted of a single cohort that would make sense. What references are you using?

As a vexillation consisted of 960 men (480 x 2), the figure of 1,000 men is rounded.

While many vexillations do seem to have been c.1000 men, and/or two cohorts, the differences in both purpose and command of vexillations suggest they were of variable size. We have one inscription to a pair of vexillations commanded by a centurion of frumentarii building a 200-ft stretch of city wall at Salona, others to vexillations repairing buildings around Hadrian's Wall, another to a vexillation of a single legion commanded by a tribune and a centurion. It would seem very unlikely that all these detachments were same size, or numbered 960 men each. Vexillations sent on active service to join a campaign army would be different to those detached as garrisons or to do construction work, but both use the same name.

I have the legionaries as consisting of vexillations, and not legions.

Quite possible - Tacitus has Agricola take 'his stand on foot before the standards' (ante vexilla) rather than before the eagles (Agricola 1.35).

However, usual practice was for the legionary force to equal the number of the auxiliaries. So if Agricola had 8000 auxiliary infantry and 3000 cavalry, his legionary strength should have been c.11,000 men. If he had four legions available, but no full legions present at the battle, each vexillation must have numbered between two and three thousand men...