We have one of the examples (paging and large message) with very low memory arguments. (50 MiB).
so, HornetQ itself has a very low footprint.
Now.. everything will depend on the number of messages you want to have pending as they stay in memory.
Look at Paging and Blocking configuration on the documentation. The MaxSize there is actually an estimate of how many bytes you want for a given address.
Or just try it yourself, just start up a server with no messages, and see how much memory it takes (e.g. using jconsole).
As Clebert says, the amount of memory required will depend on how many messages you want in memory on the server at any one time.