Lockheed Martin is currently releasing a prototype for a new range of airships that are designed from scratch to be the biggest in the world. The LMH1 is an airship capable of lifting 20 tonnes of cargo, but they also state that the design can scale up to a carrying capacity of 500 tonnes.
Why 500 tonnes? Why not 5,000? I don't know the answer to that although I suspect that it's a matter of practicability - despite the inverse square cube law applying as you rightly point out, there reaches a point where even the most powerful engines can't compete with the sail area such a volume represents. Also, helium is a valuable and non-replenishible resource; there are already many doctors out there arguing that helium should be restricted for use as a medicine so that we don't run out of it. Getting that much helium together for an airship in the future may well be the limiting factor because of the cost of the gas. That said, you wouldn't vent it to manage ballast weights, you'd just pump it back into high pressure containers for reuse, but I digress.
Of course, one of the other limiting factors is size. Once you go beyond 800m in length, the practicality of storing or hangaring such a machine can become problematic. Not to mention getting cargo to it while it's inflating its air bladder with helium, and how long it might take to in/deflate that same bladder. There is also the fact that going beyond that size actually makes mountains a hazard if it can't maneuver through ranges and the like, and while I have no hard data on this topic I suspect that the bigger it becomes, the larger the engines need to be to counter wind resistance. In other words, while the square cube law works in its advantage in terms of weight, it doesn't in terms of friction with the wind. To move such a craft through normal atmospheric pressures might take more engine power than you think.
So, if Lockheed Martin are to be believed at least, then you're looking at around 800m in length being the maximum scale of an airship, although the precise reasons why they say that is the upper bound is something you'd have to ask them directly.