Från dokumenten.
Du kan skicka batchSize med AggregationOptions i Spring Mongo 2.x version
Aggregation aggregation = newAggregation(unwind, group).withOptions(newAggregationOptions().cursorBatchSize(100).build());
Med standard batchstorlek
Aggregation aggregation = newAggregation(unwind, group).withOptions(newAggregationOptions().cursor(new Document()).build());