Reading the catalog of batteries from Panasonic, their largest-capacity flat (prismatic) cell, model UPF476790, is 3400mAh capacity with dimensions of 90mm height x 67mm width x 4.7mm thickness. You can browse their catalog of cells here which lets you sort by capacity, dimensions, weight, etc.: http://industrial.panasonic.com/www-...CA4000_WW.html

While flat batteries can make more complete use of available area inside the case, they tend to be less efficient in terms of energy density in the battery itself. So it is possible that given the same battery compartment, the 18650 with "wasted space" may still holding more energy than flat batteries which fill the whole compartment.

And, in other news, this week the Panasonic NCR18650B has been released, a single 18650 cell with 3400mAh capacity.