Late Roman Unit Sizes

[Timus]

Do you think there is any mileage in the suggestion that if Ammianus gave two clear examples of detachments numbering 300 men each were taken from the parent unit then this was a standard detachment size at the time Ammianus served in the Late Roman Army?

Possibly. Although Amm also mentions detachments of 500 on a couple of occasions, and a thousand at least once.

Regarding the possible habit of detaching 300 men from a given legion, I can't help but think of this 2 passages from Caesar BA 75:

 But this contingency had not escaped Caesar's attention: he had in fact given instruction that three hundred men out of each legion should be in light order; and these he accordingly disestablished against Labienus' cavalry to give support to his own squadrons.

Quae res Caesarem non fefellerat: namque expeditos ex singulis legionibus trecenos milites esse iusserat.

and BA 78 :

When Caesar observed these tactics he instructed the three hundred men in light order — it was his normal practice that this number of men in each of his legions should be in light order — from the legion which was posted in the line nearest the scene of this action to hasten to the assistance of the cavalry. 

Quod ubi Caesar animadvertit, CCC, quos ex legionibus habere expeditos consuerat, ex legione quae proxima ei proelio in acie constiterat iubet equitatui succurrere.

http://penelope.uchicago.edu/Thayer/E/Ro...ar/D*.html

Somebody with only a basic knowledge of the organization of a roman legion would expect a lot of numbers but not 300. Something like 60, 80, 120, 160, 480, 500, 800, 1000 would make sense but 300... ? So what are we missing ? Was there a long standing rule (from Caesar all the way to Ammianus !) of creating a legionary detachment of 300 men ? Is it just a coincidence ? Why mentioning a specific number of men rather than a specific unit level (century, maniple, cohort) ?