diff --git a/modules/reindex/build.gradle b/modules/reindex/build.gradle
index 05cd906f61160..8ea52de5a23a8 100644
--- a/modules/reindex/build.gradle
+++ b/modules/reindex/build.gradle
@@ -167,3 +167,7 @@ if (OS.current() == OS.WINDOWS) {
     }
   }
 }
+
+tasks.named("yamlRestCompatTestTransform").configure { task ->
+  task.skipTest("reindex/20_validation/specifying size fails", "size is rejected in 9.0")
+}
diff --git a/modules/reindex/src/yamlRestTest/resources/rest-api-spec/test/reindex/20_validation.yml b/modules/reindex/src/yamlRestTest/resources/rest-api-spec/test/reindex/20_validation.yml
index c96414e46f7e7..cb155bc2219ab 100644
--- a/modules/reindex/src/yamlRestTest/resources/rest-api-spec/test/reindex/20_validation.yml
+++ b/modules/reindex/src/yamlRestTest/resources/rest-api-spec/test/reindex/20_validation.yml
@@ -107,7 +107,7 @@
         body:   { "text": "test" }
 
   - do:
-      catch: /invalid parameter \[size\], use \[max_docs\] instead/
+      catch: /(invalid parameter \[size\], use \[max_docs\] instead|unknown field \[size\])/
       reindex:
         body:
           source:
diff --git a/server/src/main/java/org/elasticsearch/index/reindex/ReindexRequest.java b/server/src/main/java/org/elasticsearch/index/reindex/ReindexRequest.java
index f9d027e0c9c1c..c3e429af08a4e 100644
--- a/server/src/main/java/org/elasticsearch/index/reindex/ReindexRequest.java
+++ b/server/src/main/java/org/elasticsearch/index/reindex/ReindexRequest.java
@@ -21,7 +21,6 @@
 import org.elasticsearch.common.settings.SecureString;
 import org.elasticsearch.common.xcontent.XContentHelper;
 import org.elasticsearch.core.TimeValue;
-import org.elasticsearch.core.UpdateForV9;
 import org.elasticsearch.features.NodeFeature;
 import org.elasticsearch.index.VersionType;
 import org.elasticsearch.index.query.QueryBuilder;
@@ -355,10 +354,6 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
         );
 
         PARSER.declareInt(ReindexRequest::setMaxDocsValidateIdentical, new ParseField("max_docs"));
-
-        // avoid silently accepting an ignored size.
-        PARSER.declareInt((r, s) -> failOnSizeSpecified(), new ParseField("size"));
-
         PARSER.declareField((p, v, c) -> v.setScript(Script.parse(p)), new ParseField("script"), ObjectParser.ValueType.OBJECT);
         PARSER.declareString(ReindexRequest::setConflicts, new ParseField("conflicts"));
     }
@@ -498,10 +493,4 @@ static void setMaxDocsValidateIdentical(AbstractBulkByScrollRequest<?> request,
             request.setMaxDocs(maxDocs);
         }
     }
-
-    @UpdateForV9(owner = UpdateForV9.Owner.DISTRIBUTED_INDEXING)
-    // do we still need this ref to [max_docs] or can we remove the field entirely so it's rejected with the default message?
-    private static void failOnSizeSpecified() {
-        throw new IllegalArgumentException("invalid parameter [size], use [max_docs] instead");
-    }
 }