Premium Azure Service Bus maximum queue size is 80GB. Is there any reason that you would not set the max queue size to the maximum allowable limit?
Among the things queues are used for is to level the load. Imagine you have a burst of messages and processing service(s) cannot keep up with those messages. A queue will buffer those messages and allow processing when your service can get to the messages. Regarding 80GB, there's a little bit of history. Azure Service Bus did not have Premium tier first. The Standard tier supported a maximum size of 5GB. When a queue was partitioned, it would have exactly 16 partitions. Hence the size of 80GB, which was 16 partitions, 5GB each. When the Premium tier was introduced, it needed to support the equivalent maximum size.
What would be the case of not setting the maximum size to 80GB? Your use case. If you have messages processed quickly and they are fairly small, even 1GB would be a lot. In terms of payment, on Premium tier the size of the queue doesn't matter - you pay for the use of the premium, not the queue size. Personally, I like to limit the size of the queue not to set an expectation that a queue will become storage for messages.