What are you hoping to be able to do, using this parameter?

It wouldn't have anything to do with elephants, would it?

I must admit, I'd not paid this column too much attention, in terms of the ranges of values which already exist but I see a few possibilities.

1) It's there to enable fine tuning of how tall a unit bitmap appears, so that it only needs to be painted the once, then rescaled until it looks right in the battle environment.
2) It's supposed to reflect the height at which the unit's weapon is wielded - eg camels higher than horses, horses of two or three sizes, hobilars ("hobby horses") being the smallest and all horses higher up than foot troops and so on. Give or take polearms which are above head height and intended for bringing down horsemen. Height advantage is factored into combat results.
3) Slight misuse of the word 'height'. Looked at from above, a formation has 'height' and width. Rank spacing and a preferred number of rows factor together to fit the height parameter. If, on the other hand, three dimensions are listed, height, width, breadth, then height means what you think it means - how tall the men, or man + mount, are.

Next time I go meddling in Gnome, I'll take a closer look and see what the range of values are, as I recall seeing a heck of a lot were all the same. If it correlates with foot versus mounted units, then we'll have a clue what it means. Currently, I'm sticking with explanation (1).