ICE never set the sizes of the two armies, nor of the city of Tarahir. Those numbers were required for the history that was developed. All we needed to know was that the city would be lost, and that Grayson was put in charge of evacuating as many who were willing to leave, and getting them to safety.
As a rough guess, I would say that the city was likely somewhere around 100,000 (nowhere near as large as ancient Rome, which is thought to have had close to 1 million).
The two armies, also likely numbered in the tens of thousands (10-30 thousand, I would think.
And don't forget that this was a war being fought on two fronts, as the Orsai tried to come around both sides of that inland sea.
(note: Tarahir has been sacked/destroyed several times throughout its history the "New Tarahir" is meant to denote it from the same city when it was the capital of the Tarahiri Empire).