The full video may explain this but if someone doesn't want to watch that Wikipedia says that Aristarchus' calculations led him to conclude that the sun was 18-20 times larger than the moon. It's not clear to me whether he had a size for the moon or whether he stopped at the relative size.
QuoteAristarchus also reasoned that as the angular size of the Sun and the Moon were the same, but the distance to the Sun was between 18 and 20 times further than the Moon, the Sun must therefore be 18–20 times larger.