I take it you mean in Farads, and not the physical size.
Physically, the largest commercial capacitors are probably those found in the power industry, for example, power factor correction applications, typically seen outside factories where large electric motors are running, is my guess.
The largest single capacitor, try looking around manufacturers web sites.
The largest capacitor bank, both physically and capacitance wise, probably in a research establishment, there are claims on the web if you search. There are probably a lot tucked away that we will never hear about, since they are in military research establishments, think rail guns and lasers,
cheers,
Richard
