Mine get about 5 feet when planted in standing water. I have never had them get as large as Black Magic, I have had leaves on Black Magic in excess of 18 inches long/across

Was that in the greenhouse or outside.

Outside. All the colocasias are hardy here so I don't bother to take up any greenhouse space with those except for a few I threw into my stream just for the fun I could have "scaping" the steambed. I threw some Elepaio and Yellow Splash in the stream and a couple of "tea cups" and Nancy's Revenge into the small greenhouse pond to hang out with the Lasia Spinosa, but I also have specimens of those outdoors as well as many other colocasias.

If you want a neat colocasia that I have had grow REALLY much larger than the norm, get you hands on a Colocasia "Maui Magic". Its a Cho hybrid. I bought two stuffed full 3 gals at Lowes for $9.99 each (it sells at Plant Delights for a LOT MORE and I think Brian's Botanicals has it too on eBay) and planted them out in early summer and this baby gets LARGE. (NOT as large as something like Thai Giant or whatever, but BIG for a colocasia). I am 5"4" and these are over my head. They are beautiful too...the leaves are dark/black like a Black Magic, but shiny like a Fontanesii. Its my fave colocasia at the moment.

This plant has has shot up since i put in hot greenhouse and doesnt look like stopping any time soon, I'll keep you posted how big it gets.