I think you've got it the wrong way round.
The term 'Unit size' refers to how big each of the troops(units) of that type is.
Not how big a unit of that type you can have, since this depends on your Landing Craft's level .
Thus a Grenadier (Unit size 6) will take up 6 of the available spaces on a Landing Craft. This means only a Landing Craft level 20 (Capacity 24) can hold 4 Grenadies, and nothing can hold 6.