Slight correction here:
... it's implemented as single cache entry (i.e. the queue). This means every time you add an item to your cache, it needs to deserialize the whole queue and add the entry and then reserialize (via the publishing cache store).
The Coherence Messaging Pattern does not write or queue each entry to be queued into a single Cache Entry. Only the identities of Messages are queued, not the actual entries.
As with all deployments, the amount of memory required is based on the number of updates occurring while one or more devices (they may not be sites) are unavailable. These things should be tested not assumed. The same happens if you back your caches by disk, but have no limits on them. The amount of memory consumed is application specific.
The alternative approach (supported out-of-the-box) is to use a store-and-forward messaging provider, like something that implements the JMS specification. All releases of Push Replication are testing with both Coherence Messaging and JMS-based providers.
Hope this helps
Brian Oliver | Architect | Oracle Coherence