A standard Marian/Imperial Legion was 6000 men, but the later legions (post Constantine) contained only 1000 for a field legion of professional Comitatenses (who were comparable to traditional legionaries) or around 3000 for a border legion of the more militia-like Limitanei (who were generally spear-armed light infantry).
Both would be supplemented by a varying number and type of auxiliary units, which were typically about 800 men strong but could vary tremendously by type and location.
The 'Roman legion' that shows up is presumably a Legio Comitatenses, since the troops already available to Soissons would have been Limitanei anyway.
So it should be 1000 warriors/soldiers for the legion plus however many auxiliaries Luca wants them to have.
As far as I'm concerned after battle for Adrianopolis (IV century) legions were never used again- I hadn't read book yet, so I assumed that normal legions were used there. If it's as You say though, fine no problem. Om the other hand- late Rome forces were sign of of internal weakness of Empire. Recreating it would be great opportunity to restore old ways of military tactics (which could be successfully used in medieval too).
But Miaphisytes don't have PentarchsI'm talking about Pentarchs, sorry. The ones that are given specific bishoprics that they sit in.
One more thing- How about making (randomly, some 10% chances) invasion against Franks by Byzantium possible? In reality Justinian the Great wasn't successful enough, losing too much resources in Italy. Would he be more successful, he could invade Frankish empire and maybe even defeat it. Of course it'd fit best when bigger map is introduced, so war for Gaul can be shown. Also, if Justinian is successful, invasions from east could happen against him.