Size doesn't matter
"Others, particularly supporters of NoSQL, would argue RDMS is not suited to large systems."
The size of the system isn't the point. The nature of the data is. RDMS works well with structured data which can be at least somewhat normalized and which is queried primarily on the structured bits. RDMS does not work so well with non-structured data. NoSQL technologies, on the other hand, work better with non-structured data, but aren't as efficient as an RDMS can be with structured data.