A question about Roman Standards

[dadlamassu]

I have been reading a lot about the Roman Army and in particular that of the style of Warlord Games (et al) Early Imperial Romans. 
I have been building, off and on, a Roman force that is gradually (and unintentionally) approaching a full wargame legion.  It just keeps growing!

Anyway, I have a Signum for each cohort, an Aquila and Imago for the Legion, a Vexillum for a detachment so most of the Legionary ones are covered.

I have a Legion command group and want to add an "army Standard" type marker.  Aquila goes with the First Cohort so what would the commander have?  I have read (somewhere) that when several legions were deployed on a single operation the commander had his staff, escort, trumpeters etc and a standard that identified him and the army in purple letters.  Whether or not the letters were purple is less important to me than finding out what type of standard this was. 

I have my own idea that it was probably a vexillum, possibly larger than the one used to mark a detachment (Vexillation).  But what might it actually have looked like? 

Anyone know?  Anyon got any ideas?

[dadlamassu]

(I found the reference) the type I looking for is mentioned by Cassius Dio (Roman History, III, xl, 18) "But one of the large flags, that resemble sails, with purple letters upon them to distinguish the army and its commander in chief, was overturned ... in a violent wind.  Crassus had the others of equal length cut down so they might be shorter and steadier to carry"

[wmyers]

I’ve never seen a standard like that sail represented in a game or in miniatures.

[dadlamassu]

I’ve never seen a standard like that sail represented in a game or in miniatures.

Nor have I until the labarum used by Constantine and I have only found the one reference in Dio.  It may be that the labarum was the Christian a follow on to this large vexillum?