Most of the Monero size forecasts you see are anti-Monero FUD. They rely on a linear extrapolation of the current size which is greatly inflated by the old pool software that ran in the first few months. The actual growth rate is a fraction of that estimate.

However, I agree with the other comments that 57 GB or even larger is not a big deal given that today you can easily buy a 1 TB HD or even a 64 GB microSD which I recently priced at $12

As long as the database is designed with scalability in mind, this sort of space requirements is not an issue.

I think 57GB will be the new 57MB soon enough:

link

57 GB by 2020 is completely doable, the people who want to be their own bank should go buy a 1TB hard drive.

Of course it is going to get big if it is used by billions of people. How could it possibly be otherwise?

As far as 57 GB by 2020, or any other such guess, I have absolutely no idea. It entirely depends how many people are using it and how much.