I`m not sure what is the smallest, but I have a Silverstone SUGO SG09. There is also a SG10 now, which looks better.
The case is smaller than a Mini-ITX Bitfenix Prodigy, and others.
I have also a 212 Evo in there, and a 3-slot R9 280x Matrix, but the R9 280x Toxic, which is a lot longer, will also fit.
Any case you use, go for modular PSU, and if you get a Silverstone PSU, also the PP05 cables (shorter than normal) for good airflow and no cable clutter.