The Hail Caesar army lists are guidelines, not gospel.
On page 41 in the Camillan Rome section it says that:
"If players prefer to to field units at standard size they can certainly do so and suitable stats for full-size units can be extracted from the Marian list."
And on page 53 in the Republican Rome section it says:
"As mentioned in the Camillan list we would not quibble with players who prefer to deploy their units as standard sized rather than small if that is their preference, the Marian list has suitable stats for such units."
There are some distinct advantages to fielding Republican Roman legionaries as small units, such as the ability to fight 2-to-1 against standard-sized enemy units. But ultimately, how you choose to field them is entirely up to you.
Hope this helps!
“What we wish, we readily believe, and what we ourselves think, we imagine others think also.”
~ Gaius Julius Caesar