I'm not sure what you are getting at, but let me say this: If you have a device configuration with a graphene 2D sheet that is periodic along B, has vacuum along A, and has C as its transport direction, it is very reasonable to divide the current with the width of the sheet along B.
Why is this? An example: Let's say you have run the calculation, and the current is 500 nA. However, your graphene sheet is in principle infinite along B. Does this mean that the current through a macroscopic graphene sheet is 500 nA * inf = inf ? No, that clearly makes no sense. Truly infinite 2D sheets do not exist. Much better then to think of an intrinsic property such as current density: 500 nA/0.5 nm = 1000 nA/nm.