So how accurate are the unit sizes for roman legions?

Say, the 80 men you get in a unit of roman infantry in large settings.

My first instinct was that it should have been 100, but reading up on it it seems a century had 80 men.

(And a cohort was six or something centuries).

It seems each century had it's own centurion and standard bearer? At any rate, I've added in a centurion and standard bearer to my hastati and principes, because it just looks better :)

And then there are these things called maniples, or double centuries, which apparently had 160 men?

That would be great if the default sizes for roman legionary units matched up historically :)

I don't know too much about the subject ( just what I have read online and in a book a long time ago) so I'm eager to know if they are accurate or if I'm just mistaken.

The problem is that everything is still scaled about 10:1--well all except elephants and some other special units. So you can't form full legions the same way. It is more important to have relative scale make sense. The 120 man Macedonian phalanxes make sense vs. 80 man Roman units because of frontage, facing effects, unit cohesion, etc. Some others make less sense like the 80 man phalanx units. Triarii are a bit oversized relative to hastati and principes, but with the way they are trained and the way the battle system works, that is not really an issue.

In the Post Marian army, 80 men were one century. Two centuries made a maniple, six centuries made a cohort. Ten cohorts made a legion. Now, realize, this is all on paper. On campaign, the numbers would certainly be different due to disease, or distrust of their commander.

Which you've already said. The centuries did have their own centurions and standard bearers, but these standard bearers aren't those in the game. The ones in game seem to be aqulia (I may be wrong here), which shouldn't be covered in anmial skins, and should be with the first cohort. The standard bearers I see do have bearskins, but they carry a staff of discs, to signify which cohort they belonged to.

The standard bearers that principes and triarii have in the game are signifers. Aquilifers, IIRC, are a Marian addition. As I have not triggered the Marian reforms yet I have not seen them, if they are modelled at all. There was but one per legion.