according to Sun Tzu size comes in numbers, not weaponery
300 marines even with machine guns wouldn't be enough to take down 60,000 romans. first, they'd be way too intimidated by the sheer size of the roman army, which would obviously cause panic and morale to deteriorate instantly. second, the amount of bullets required to sustain 300 marines continuously firing on that many romans would cost millions of dollars and isn't even realistic in a military sense.
Most assault rifles would only be effective at around 100 meters and by that time, the romans can easily charge and close that distance enough to charge the marines. Once the romans are within 20 meters then the marines are essentially defenseless anyways and would probably run
300 marines can probably hold their ground against an army 3x their size if they were stationery, but if you gave them horses for example, they'd probably be able to fight against an army 20x their size