The size of the JVM heap really depends on the type of Druid node you are running. Below are a few considerations.
Broker nodes uses the JVM heap mainly to merge results from historicals and real-times. Brokers also use off-heap memory and processing threads for groupBy queries.
Historical nodes use off-heap memory to store intermediate results, and by default, all segments are memory mapped before they can be queried. Typically, the more memory is available on a historical node, the more segments can be served without the possibility of data being paged on to disk. On historicals, the JVM heap is used for GroupBy queries, some data structures used for intermediate computation, and general processing. One way to calculate how much space there is for segments is: memory_for_segments = total_memory - heap - direct_memory - jvm_overhead.
[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.