57 million tons is equal to 114 billion pounds. According to this Wikipedia page the average adult in North American weighs 180lbs.
114,000,000,000 lbs / 180 lbs/person = 633,333,333 people
So it’s an amount of plastic that weighs as much as 633 million people; of course it is still impossible to really imagine how much plastic that is. But maybe a better reference then the volume a park.