Just for reference, MB transitioned the def tank from the passenger side storage area (2011), to being contained on the Sprinter chassis under the hood, in the engine compartment (2012) between the 2011 and 2012 model year.
To determine exact chassis year, you need to look at your VIN#, of which the 10th position determines mfg'd year. So, as a reference the letter C = 2012, and as the alphabet increases, so does the chassis mfg. years' date. The capacity of the def tank for 2012 is exactly 12 liters. I'd assume that continues to be the same for the subsequent years.
Hope that helps.