Redis is an in-memory data store, that can persist it’s stated to disk (to enable recovery after restart). However, being an in-memory data store means the size of the data store (on a single node) cannot exceed the total memory space on the system (physical RAM + swap space). In reality, it will be much less than this, as Redis is sharing that space with many other processes on the system, and if it exhausts the system memory space it will likely be killed off by the operating system.
Mongo is a disk-based data store, that is most efficient when it's working set fits within physical RAM (like all software). Being a disk-based data means there are no intrinsic limits on the size of a Mongo database, however configuration options, available disk space, and other concerns may mean that databases sizes over a certain limit may become impractical or inefficient.
Both Redis and Mongo can be clustered for high availability, backup and to increase the overall size of the datastore.