What would be the standard vexillation size?

[Frank]

A mathematical pattern in divisions of 1,000 men emerges.

Scheuerbrandt analyzed the roman army extensively in his dissertation "Exercitus". He found, that the romans usually thought in figures like 500, 1000, 2000, 4000, 8000, 16.000 and 32.000 men, according to the so called "greek-tacticians". Not in 1 legion, 2 legions, ... plus auxilia. 500 is a known standard size (cohort, ala). Vespasian established smaller armies (vexillationes?) of 2000 men (1000 heavy infantry, 500 light infantry, 500 cavalry) in order to pacify the several regions of Iudaea finally. At the limes in Germania Scheuerbrandt identified sections secured by 2000 men, again composed of multiple units commanded by a tribunus. So, yes 1000 and 2000 was a usual size for roman armies and often used by commanders.