The largest free DB I know is Planet data by OpenStreetMap
http://planet.openstreetmap.org/
However, if you just need random data in format you need, I would suggest writing scripts/seeders and generate million-billion of random rows. I once waited an hour for generating tens of millions of rows on MySQL on Home PC to do some benchmarks and test different data structures.
However, your main problem is not about data, it is about "What DB to choose" and the answer is simple - use what you know the best until someone in your team will propose a better solution based on his real experience for your real business use case. I am using MySQL because I know it the best, because of good administrator for it - MySQL Workbench and because MySQL 5.7 had significant performance improvements, NoSQL support and many other features, the upcoming MySQL 8 is even more awesome. I am happy with that Oracle started developing MySQL for last years and they now provide a lot of modern enterprise solutions including MySQL in cloud, load balancing and scale.
Stop thinking about performance from the day 1. You, probably, will never have even a million of rows for the next 10 years. For side projects, there is no difference, choose what you know the best and feel comfortable yourself with.