diff --git a/services/document-updater/app/js/RedisManager.js b/services/document-updater/app/js/RedisManager.js
index 73d85f60d5..418e3ec6d4 100644
--- a/services/document-updater/app/js/RedisManager.js
+++ b/services/document-updater/app/js/RedisManager.js
@@ -72,6 +72,13 @@ module.exports = RedisManager = {
       logger.error({ err: error, doc_id, docLines }, error.message)
       return callback(error)
     }
+    // Do a cheap size check on the serialized blob.
+    if (docLines.length > Settings.max_doc_length) {
+      const docSize = docLines.length
+      const err = new Error('blocking doc insert into redis: doc is too large')
+      logger.error({ project_id, doc_id, err, docSize }, err.message)
+      return callback(err)
+    }
     const docHash = RedisManager._computeHash(docLines)
     // record bytes sent to redis
     metrics.summary('redis.docLines', docLines.length, { status: 'set' })
@@ -461,6 +468,13 @@ module.exports = RedisManager = {
         logger.error({ err: error, doc_id, newDocLines }, error.message)
         return callback(error)
       }
+      // Do a cheap size check on the serialized blob.
+      if (newDocLines.length > Settings.max_doc_length) {
+        const err = new Error('blocking doc update: doc is too large')
+        const docSize = newDocLines.length
+        logger.error({ project_id, doc_id, err, docSize }, err.message)
+        return callback(err)
+      }
       const newHash = RedisManager._computeHash(newDocLines)
 
       const opVersions = appliedOps.map((op) => (op != null ? op.v : undefined))
diff --git a/services/document-updater/app/js/ShareJsUpdateManager.js b/services/document-updater/app/js/ShareJsUpdateManager.js
index 607ae2d9fa..8ae91df32c 100644
--- a/services/document-updater/app/js/ShareJsUpdateManager.js
+++ b/services/document-updater/app/js/ShareJsUpdateManager.js
@@ -87,6 +87,20 @@ module.exports = ShareJsUpdateManager = {
         if (error != null) {
           return callback(error)
         }
+        const docSizeAfter = data.snapshot.length
+        if (docSizeAfter > Settings.max_doc_length) {
+          const docSizeBefore = lines.join('\n').length
+          const err = new Error(
+            'blocking persistence of ShareJs update: doc size exceeds limits'
+          )
+          logger.error(
+            { project_id, doc_id, err, docSizeBefore, docSizeAfter },
+            err.message
+          )
+          metrics.inc('sharejs.other-error')
+          const publicError = 'Update takes doc over max doc size'
+          return callback(publicError)
+        }
         // only check hash when present and no other updates have been applied
         if (update.hash != null && incomingUpdateVersion === version) {
           const ourHash = ShareJsUpdateManager._computeHash(data.snapshot)
diff --git a/services/document-updater/test/acceptance/js/SizeCheckTests.js b/services/document-updater/test/acceptance/js/SizeCheckTests.js
new file mode 100644
index 0000000000..288cc485e1
--- /dev/null
+++ b/services/document-updater/test/acceptance/js/SizeCheckTests.js
@@ -0,0 +1,129 @@
+const { expect } = require('chai')
+const Settings = require('settings-sharelatex')
+
+const MockWebApi = require('./helpers/MockWebApi')
+const DocUpdaterClient = require('./helpers/DocUpdaterClient')
+const DocUpdaterApp = require('./helpers/DocUpdaterApp')
+
+describe('SizeChecks', function () {
+  before(function (done) {
+    DocUpdaterApp.ensureRunning(done)
+  })
+  beforeEach(function () {
+    this.version = 0
+    this.update = {
+      doc: this.doc_id,
+      op: [
+        {
+          i: 'insert some more lines that will bring it above the limit\n',
+          p: 42
+        }
+      ],
+      v: this.version
+    }
+    this.project_id = DocUpdaterClient.randomId()
+    this.doc_id = DocUpdaterClient.randomId()
+  })
+
+  describe('when a doc is above the doc size limit already', function () {
+    beforeEach(function () {
+      this.lines = ['0123456789'.repeat(Settings.max_doc_length / 10 + 1)]
+      MockWebApi.insertDoc(this.project_id, this.doc_id, {
+        lines: this.lines,
+        v: this.version
+      })
+    })
+
+    it('should error when fetching the doc', function (done) {
+      DocUpdaterClient.getDoc(this.project_id, this.doc_id, (error, res) => {
+        if (error) return done(error)
+        expect(res.statusCode).to.equal(500)
+        done()
+      })
+    })
+
+    describe('when trying to update', function () {
+      beforeEach(function (done) {
+        const update = {
+          doc: this.doc_id,
+          op: this.update.op,
+          v: this.version
+        }
+        DocUpdaterClient.sendUpdate(
+          this.project_id,
+          this.doc_id,
+          update,
+          (error) => {
+            if (error != null) {
+              throw error
+            }
+            setTimeout(done, 200)
+          }
+        )
+      })
+
+      it('should still error when fetching the doc', function (done) {
+        DocUpdaterClient.getDoc(this.project_id, this.doc_id, (error, res) => {
+          if (error) return done(error)
+          expect(res.statusCode).to.equal(500)
+          done()
+        })
+      })
+    })
+  })
+
+  describe('when a doc is just below the doc size limit', function () {
+    beforeEach(function () {
+      this.lines = ['0123456789'.repeat(Settings.max_doc_length / 10 - 1)]
+      MockWebApi.insertDoc(this.project_id, this.doc_id, {
+        lines: this.lines,
+        v: this.version
+      })
+    })
+
+    it('should be able to fetch the doc', function (done) {
+      DocUpdaterClient.getDoc(
+        this.project_id,
+        this.doc_id,
+        (error, res, doc) => {
+          if (error) return done(error)
+          expect(doc.lines).to.deep.equal(this.lines)
+          done()
+        }
+      )
+    })
+
+    describe('when trying to update', function () {
+      beforeEach(function (done) {
+        const update = {
+          doc: this.doc_id,
+          op: this.update.op,
+          v: this.version
+        }
+        DocUpdaterClient.sendUpdate(
+          this.project_id,
+          this.doc_id,
+          update,
+          (error) => {
+            if (error != null) {
+              throw error
+            }
+            setTimeout(done, 200)
+          }
+        )
+      })
+
+      it('should not update the doc', function (done) {
+        DocUpdaterClient.getDoc(
+          this.project_id,
+          this.doc_id,
+          (error, res, doc) => {
+            if (error) return done(error)
+            expect(doc.lines).to.deep.equal(this.lines)
+            done()
+          }
+        )
+      })
+    })
+  })
+})