Late Roman Unit Sizes

[Nathan Ross]

25.000 - the army of Galerius in Persia, 298 AD (Festus Brev. XXV)

This was Galerius's second force, and it was apparently formed or reinforced by troops drawn from the Danube legions, and also a substantial group of Gothic allied troops. The others you mention (I think) are also specific expeditions made up of troops drawn from other places.

My point was that Terentius was probably using the establishment force available to him as a provincial dux - quite possibly very similar to the numbers and composition available two centuries earlier to a governor like Arrian.

the term 'legion' could aslo already have become a generalitic description of a Roman infantry unit, which even continues to the British monk Gildas in the early 6th century.

Yes, quite! I don't think there's any doubt that later writers are using 'legion' to mean a much smaller unit than those of the principiate. Excepting Vegetius, with his interesting 6000-man 'Herculiani' legion...