If the CRX-based Communiqué instance is used to store loads of large documents the data volume inside the repository will grow. Is there a maximum size the CRX is capable of handling? Or is there any information regarding how repository size affects performance?
The volume of data that can be stored in CRX is not a limitation factor of CRX itself but of the underlying persistence layer, be it a filesystem or a database. Therefore you need to consider limitations and potential known issues of the persistence layer in question.
Same applies for performance: factors such as network latencies and overhead when using remote database servers and filesystems need to be taken into account.
Regarding CQ/CRX itself, the only known limitation that might affect performance directly are flat hierarchies. This means that you should avoid having content structures with more than 1000 nodes on the same level in the hierarchy.