What Franklin means in this quote is that a person who is truly great will act as if he is an equal to all men. When I teach about the Revolutionary War, I often use this quote to talk about what it was that the Patriots were trying to accomplish in terms of remaking society.
Franklin was saying that all people should act is if they are equal. A person who is truly great will do this. Such a person will not step on a worm -- that is, he will not use his power to abuse those who are lower than him. Instead, he will treat them as equals. At the same time, he will not suck up to those above him. Instead, he will treat them with respect, but still as his equals.
This was the sort of attitude that made America a country that valued equality much more than Britain and other Old World countries did at that time.