How do I drill down into the storage usage of my cluster?

From Marvel, I can tell that my cluster contains 240 million docs and 640GB. I'd like to drill down further into this, asking questions like:

  • What is the average document size?
  • What are the largest documents?
  • Which fields occupy the most space?

I've tried to run an aggregation to answer the first question:

GET /profound_v2/urls/_search
  "query" : {"match_all" : {}},
  "aggs" : {"avg_size" : {"avg" : {"terms" : {"field" : "_size"}}}}

But I get an error:

  "root_cause": [
        "type": "search_parse_exception",
        "reason": "Unexpected token START_OBJECT in [avg_size].",
        "line": 3,
        "col": 46

Could someone please put me on the right track?

I've moved this to the Elasticsearch category since it is not specific to Elastic Cloud.