Roman Cavalryman Height?

[Pompeius Minus]

I know that during the Napoleonic period they grouped troops of the same height together in companies...

In the Napoleonic era they actually had both minimum and maximum height requirements. Grenadiers, guardsmen and heavy cavalry had a mimimum height, while skirmishers and light cavalry had a maximum height.

The height requirement for the grenadiers, i.e. the tallest soldiers, in Napoleon's imperial guard in 1815 was only 1.65 m or 5 feet 5 inches. This again makes me doubt that the Romans could find enough soldiers that were 5 feet 10 inches (modern measurements) or 1.77 m tall or above to fill all their first cohorts.