Whatever big is defined as, it needs to at least be that your data can't fit into RAM on a high end server.
There's also the threshold where your indexes don't fit into RAM. And the threshold where your data no longer fits into PCIe SSDs on a single server. (The combined bandwidth of the SSDs will rival the RAM, but with more latency.)
There's also the threshold where your indexes don't fit into RAM. And the threshold where your data no longer fits into PCIe SSDs on a single server. (The combined bandwidth of the SSDs will rival the RAM, but with more latency.)