Biggest I've seen is about 1.3 TB....sql server had 64 cores, 64gb of RAM and some pretty fast storage, mind you.
Multi-threading in the data engine is a must in these sort of throughputs (insert rate was approx 6500 msg/sec if my memory serves me right)
The speed of the storage is probably the most important factor, or rather the likely bottleneck.
Then there's always multiple data engines....