The size of the JVM heap really depends on the type of Druid node you are running. Below are a few considerations.
Broker nodes can use the JVM heap as a query cache and thus, the size of the heap will affect on the number of results that can be cached. Broker nodes do not require off-heap memory and generally, heap sizes can be set to be close to the maximum memory on the machine (leaving some room for JVM overhead). The heap is used to merge results from different real-time and historical nodes, along with other computational processing.
Historical nodes use off-heap memory to store intermediate results, and by default, all segments are memory mapped before they can be queried. The more off-heap memory is available, the more segments can be served without the possibility of data being paged onto disk. On historicals, the JVM heap is used for GroupBy queries, some data structures used for intermediate computation, and general processing.
[Coordinator nodes](Coordinator nodes) do not require off-heap memory and the heap is used for loading information about all segments to determine what segments need to be loaded, dropped, moved, or replicated.
The intermediate computation buffer specifies a buffer size for the storage of intermediate results. The computation engine in both the Historical and Realtime nodes will use a scratch buffer of this size to do all of their intermediate computations off-heap. Larger values allow for more aggregations in a single pass over the data while smaller values can require more passes depending on the query that is being executed. The default size is 1073741824 bytes (1GB).
Server maxSize sets the maximum cumulative segment size (in bytes) that a node can hold. Changing this parameter will affect performance by controlling the memory/disk ratio on a node. Setting this parameter to a value greater than the total memory capacity on a node and may cause disk paging to occur. This paging time introduces a query latency delay.