Evidently Bad is MIA on the juice capacity question, I'd be interested in an exact number, but I'm guessing maybe 2.5ml for the standard length 19mm? Sound right?
If you compare an IBtank to the ego-diameter Smoktech DCTs, which are rated at like 3.5ml or something (I'm assuming that includes the carto's capacity, as a 3ml bottle of juice is just about the same size of a tank.) you'll notice that the inside diameter of the tank's tube is the same inner diameter of the plastic tube on the slim dcts. The tank is fat because of how thick the glass is, if using a thin plastic tube the capacity would probably double.
They actually probably hold a little less than the slim dcts, since those take XL cartos so the tank is taller. You can always get the xl length one.
They hold less than they look like they should, bud I'll agree with adeline, for me it's "enough"