To my knowledge the cluster built by GCHQs is the currently the largest one. Does anyone know of a larger cluster currently being created? I found a 300 Pi cluster being built by this group http://megarpi.inf.unibz.it/index.php/about-us
We can't really tell since anyone (including me) might have already built a 5000-node cluster and simply did not publish their work.
The largest one that's made aware to the public is, in fact, the GCHQ cluster.
Update: This(link) is bigger.
I might have a 10,000 node Pi 3 cluster in my secret Pacific ocean underwater lair. Muhahahahahha!
this is bigger at 750 raspberry pi 3s at 3000 cores total
Not strictly Raspberry Pi cluster, but still a huge 1 million ARM cores cluster to simulate the human brain. https://www.zdnet.com/article/million-core-arm-machine-aims-to-simulate-brain/