Ok.. let's take the example of a Cohors Reformata, which consists of 480 men. Under 1:2 scale, it would boost the unit up to it's maximum actual size, 240.

In the EDU:
soldier roman_infantry_cohorsreformata, 50, 0, 1.18
would become
soldier roman_infantry_cohorsreformata, 60, 0, 1.18

In 1:3 scale, it would be 160 men, thus:
soldier roman_infantry_cohorsreformata, 40, 0, 1.18

In 1:4 scale, it would be 120 men, thus:
soldier roman_infantry_cohorsreformata, 30, 0, 1.18

In 1:5 scale, it would be 96, thus:
soldier roman_infantry_cohorsreformata, 24, 0, 1.18

And so on. 1:10 gives us quite useful proportions even at this point, being 48 men and thus:
soldier roman_infantry_cohorsreformata, 12, 0, 1.18

The script would need to edit both the SP and MP EDUs (or leave an option to specify which, if that would be unviable) and would display the max men you can theoretically field (going by scales, of course, as the actual number of men on the battlefield will not change visually). So, you could easily switch between 1:1 scale (when you'd like to enjoy something around the size of a single Marian Legion) and 1:10 scale (when you'd like to enjoy things on a truly gargantuan scale, even the "exaggerated" antiquity battles, including a battle I remember hearing about where Roman troops are said to have faced 250,000 Britons).
I dont think this is very good idea.

First you would need to find biggest formation on battlefield to make sure it will fit into 240 men unit ( on huge ).
And you must remember that we dont have alot of data for such calculations.
So lets take from wiki for example 1,500-man Macedonian phalanx brigade.
To fit it in 240 unit you must use 1:7 scale.

This means 214men phalanx and your Cohors Reformata is down to only 68 men on Huge!