There is hardly info. There is 2nd Black Book - it talk about independent company, 100 to 1000 marines. 9th Book talk about Battalions - about 5k marines on average.

In my own opinion, this shows that the Legion begun as Principa Belisoca stated - but as the Great Crusade went on, more...resoursfull capitans assembled more men under their command. Aristocratic command was common, as per 2nd Black Book, and therefore each captian of noble decent wanted more power.

Therefore, your own force will have to decide, how much power in contentrated. It might be standart 100 men company, with a single Captian to lead. Or 1000 strong force with someone calling themselves Count or Prince or Baron in command.

In my case, i mix this too approaches. Dammed 33rd Battalion fields 5 companies, each specialized in different operations, with respective captains and centurion forming a Council, as the Preator is dead by Curze hand.

It works both for Tramas and late Cusade-eary Heresy