~]$ ./repro.sh
Creating "foo" and sharding by "geo" (mongos = mongos_svr_A:27017)
{
        "_id" : "reshard_test_db.foo",
        "lastmodEpoch" : ObjectId("5bf3bd2e932b8fad2c4cf328"),
        "lastmod" : ISODate("1970-02-19T17:02:47.296Z"),
        "dropped" : false,
        "key" : {
                "geo" : 1
        },
        "unique" : false,
        "uuid" : UUID("c4e4f1ac-76cb-4c79-b816-91b4ee6b11ac")
}

Running dataSize (mongos = mongos_svr_B:27017)
{  "dataSize" : "reshard_test_db.foo",  "keyPattern" : {  "geo" : 1 },  "min" : {  "geo" : 100 },  "max" : {  "geo" : 200 } }
{
        "size" : 3500,
        "numObjects" : 100,
        "millis" : 0,
        "ok" : 1,
        "$clusterTime" : {...},
        "operationTime" : Timestamp(1542700334, 418)
}

Dropping "foo", recreating and re-sharding by "_id"
{
        "_id" : "reshard_test_db.foo",
        "lastmodEpoch" : ObjectId("000000000000000000000000"),
        "lastmod" : ISODate("2018-11-20T07:52:14.806Z"),
        "dropped" : true
}

Running dataSize (mongos = mongos_svr_B:27017)
{  "dataSize" : "reshard_test_db.foo",  "keyPattern" : {  "_id" : 1 },  "min" : {  "_id" : 100 },  "max" : {  "_id" : 200 } }
{
        "ok" : 0,
        "errmsg" : "keyPattern must equal shard key",
        "code" : 13408,
        "codeName" : "Location13408",
        "$clusterTime" : {...},
        "operationTime" : Timestamp(1542700336, 52)
}

Executing an insert() on foo. (mongos = mongos_svr_B:27017)
WriteResult({ "nInserted" : 1 })

Running dataSize again (mongos = mongos_svr_B:27017)
{  "dataSize" : "reshard_test_db.foo",  "keyPattern" : {  "_id" : 1 },  "min" : {  "_id" : 100 },  "max" : {  "_id" : 200 } }
{
        "estimate" : false,
        "size" : 1800,
        "numObjects" : 100,
        "millis" : 0,
        "ok" : 1,
        "$clusterTime" : {...},
        "operationTime" : Timestamp(1542700336, 54)
}