You guys are talking past each other, as your workloads appear to be different. With a traditional RDBMS, size usually wouldn't be a bottleneck, as long as you can add enough disks to a single server.

Write operations per second, that's the metric I would care about. A 50TB instance with low amount of write operations can be zero maintenance, while a 500GB instance with high amount of write operations can be a real pain.