Whoa, I can't believe I missed this one!

Anyhow, others have done a good job of describing Roman military units at
the dawn of the empire, their equipment, etc. However, as someone
mentioned, Roman Legions are rather large for a Birthright campaign (being
over 5,000 strong if around full strength.)
Here's a OOB I created (actually for Mystara/Thyatis) that uses a 2,000 man
'Legion'. It also reflects a late-Roman military system (in that cavalry is
integrated with infantry to form a "combined arms" division.):

Division Name: I prefer Exercitus (the late Roman designation for "army")
to Legion (why? 'cause everyone uses "Legion" for everything.)

Division Command Staff: Legatus Legio [or Exercito] (commander), Tribunus
Laticlavius (Deputy Commander), Primus Pilus (Senior Centurion), 1 Mage
Arcanus (senior Mage), 1 Prelate (Legion Priest), 5 Tribuni Augusticlavii,
1 Eagle Bearer (carrying the Legion Standard), 1 Draconarius (carrying the
Dragon Symbol), 1 Engeneer, 1 Alchemist, 2 aides (Spatharii) [to the Leg.
Leg. & Trib. Lati.]

1st & 2nd Cohort [or Numerus]: 1Tribunus Cohortis (commander), 1
Praefectus Cohortis (Deputy commander), 1 Mage (or, in BR, magician or
other specialist), 1 Priest.
1st-3rd Century [or Bandon]: 1 Centurion, 1 Spatharius, 120 Medium Cavalry
(Lamilar or Banded armor, Buckler, lance, longsword [Spatha], short
composite bow, medium war horse with half chain barding.)
4th Century [or Bandon]: 1 Centurion, 1 Spatharius, 120 Light Cavalry
(leather armor, short composite bow, lance, longsword [spatha], light war

3rd & 4th Cohort [or Numerus]: 1 Tribune Cohortis, 1Praefectus Cohortis, 1
Mage (in Br magician or other specialist), 1 Priest.
1st-3rd Century [or Bandon]: 1 Centurion, 1 Spatharius, 120 Heavy Infantry
(Banded Mail, large shield, pike, short sword [Gladius], javelins [Pilia].
4th Century [or Bandon]: 1 Centurion, 1 Spatharius, 120 Archers (leather
armor, long composite bow, short sword [Gladius].

Gives a BR OOB of 8-16 war cards (depening on whether your legions
represent efficient, quality troops [16], or decadent levys [8].) Several
people in the division would have the Siegecraft skill (representing the
Roman Legion's ability to construct siege engines on the spot; they carried
the metal components with them, and then used local materiels to build the
engines. N.B.: just as in the tabletop game "Imperium Romanum II", there
should be "deforested" provinces [I.E. the Khinasi plains] where this is
not possible.) In addition your "Rome" should have some specialized, elite
Artillerist units (representing Rome's "big guns"), as well as Imperial
Guards (Praetorians, Palatine Guards, Scholae, or whatever.)

Tactics is a whole big topic. This particular Legion however is designed to
deploy as follows: Infantry in the middle (archers behind, or intersperced
between, Heavy Cavalry). Cavalry covering the Flanks. Light Cavalry
deployed slightly in advance of the Legion as a whole (as scouts and
skirmishers.) There are a number of neat tricks I've developed using this
formation (usually having to do with disguising/hidding part of the
cavalry, and envelopment.) N.B.: of course this is not strictly speaking
"pure historical", and represents components of several periods of Roman
history. But it is game-functional. (If you're interested, I could probably
come up with "War Card" stats for this formation. there are really only 4
different types of troops, representing the Roman drive for
Also, I've left out special auxillaries (slingers, barbarian cavalry,
scouts, etc) that should be attached to any legion (thus, you get 12-20 war
cards or so for each one. . .you're still talking "big" for B.R. but Rome
isn't some petty Vulgarian fiefdom.)

Equipment: In addition to the weapons (listed above), each soldier should
have the following: Waterproof (heh) felt cloak (in unit colors), helmet
(with plume of unit color), Tunic (linen or wool, especially in hot
environs, to keep the sun from heating your armor), dagger and hand axe
(more used as tools than weapons.)
Each squad would have: round, roomy felt tent (in the nomad style),
sickles, axes, hammer, shovels, and other tools. They'd carry several
stakes (already cut to size, for use in constructing the palaside that
surrounds every legion's fortified camp), and a servant (lightly armed;
together the servants would provide [slight] protection of the camp when
the legion goes to battle. Mainly they do the mundaine chores (I.E. getting
the meal ready while the legionare's build the camp for the night.)
Camp followers: the traditional Roman Legion didn't take the usual gaggle
of hangers on ("party girls", wives, brats, hucksters, etc.) that followed
most armies of the middle ages (I.E. the Crusaders.) For this reason (above
all others), they were significantly faster than such rag-tag armies. (this
is hard to replicate in B.R., but I'd give "Rome" a slight, but
significant, initiative bonus (say, +1) every turn, to represent the
relative efficiency of it's bureocracy & military.)
Btw, speaking of that Bureocracy; if you need non-military ranks (I.E.
dudes to replace counts n dukes n such. . .er, wait, thouse are Roman
titles that got transmogrified into fudal ones). Well, anyhow, if you need
titles of dudes that fill out the Roman civil structure (I.E. instead of
chamberlain, what?), let me know. I don't wanna type all that up right yet,
specially if it isn't needed.