diff --git a/beacon_chain/ssz.nim b/beacon_chain/ssz.nim
index a70884cfe..62466dcf5 100644
--- a/beacon_chain/ssz.nim
+++ b/beacon_chain/ssz.nim
@@ -206,8 +206,7 @@ proc writeVarSizeType(w: var SszWriter, value: auto) {.raises: [Defect, IOError]
 
   when T is seq|string|openarray:
     type E = ElemType(T)
-    const isFixed = when E is Option: false
-                    else: isFixedSize(E)
+    const isFixed = isFixedSize(E)
     when isFixed:
       trs "WRITING LIST WITH FIXED SIZE ELEMENTS"
       for elem in value:
@@ -243,6 +242,32 @@ proc writeValue*(w: var SszWriter, x: auto) {.gcsafe, raises: [Defect, IOError].
   else:
     unsupported type(x)
 
+func sszSize*(value: auto): int =
+  mixin toSszType
+  type T = type toSszType(value)
+
+  when isFixedSize(T):
+    anonConst fixedPortionSize(T)
+
+  elif T is seq|string|array|openarray:
+    type E = ElemType(T)
+    when isFixedSize(E):
+      len(value) * anonConst(fixedPortionSize(E))
+    else:
+      result = len(value) * offsetSize
+      for elem in value:
+        result += sszSize(toSszType elem)
+
+  elif T is object|tuple:
+    result = anonConst fixedPortionSize(T)
+    enumInstanceSerializedFields(value, _, field):
+      type FieldType = type toSszType(field)
+      when not isFixedSize(FieldType):
+        result += sszSize(toSszType field)
+
+  else:
+    unsupported T
+
 proc writeValue*[T](w: var SszWriter, x: SizePrefixed[T]) {.raises: [Defect, IOError].} =
   var cursor = w.stream.delayVarSizeWrite(10)
   let initPos = w.stream.pos
@@ -465,7 +490,7 @@ func merkleizeSerializedChunks(merkleizer: SszChunksMerkleizer,
       # We assume there are no side-effects here, because the
       # SszHashingStream is keeping all of its output in memory.
       hashingStream.writeFixedSized obj
-      hashingStream.flush
+      close hashingStream
     merkleizer.getFinalHash
   except IOError as e:
     # Hashing shouldn't raise in theory but because of abstraction
diff --git a/tests/official/fixtures_utils.nim b/tests/official/fixtures_utils.nim
index 8fa440d7b..0d74f2708 100644
--- a/tests/official/fixtures_utils.nim
+++ b/tests/official/fixtures_utils.nim
@@ -34,6 +34,10 @@ proc readValue*(r: var JsonReader, a: var seq[byte]) {.inline.} =
 # #######################
 # Test helpers
 
+type
+  UnconsumedInput* = object of CatchableError
+  TestSizeError* = object of ValueError
+
 const
   FixturesDir* = currentSourcePath.rsplit(DirSep, 1)[0] / ".." / ".." / "vendor" / "nim-eth2-scenarios"
   SszTestsDir* = FixturesDir/"tests-v0.11.1"
@@ -47,3 +51,15 @@ proc parseTest*(path: string, Format: typedesc[Json or SSZ], T: typedesc): T =
     stderr.write $Format & " load issue for file \"", path, "\"\n"
     stderr.write err.formatMsg(path), "\n"
     quit 1
+
+template readFileBytes*(path: string): seq[byte] =
+  cast[seq[byte]](readFile(path))
+
+proc sszDecodeEntireInput*(input: openarray[byte], Decoded: type): Decoded =
+  var stream = unsafeMemoryInput(input)
+  var reader = init(SszReader, stream)
+  result = reader.readValue(Decoded)
+
+  if stream.readable:
+    raise newException(UnconsumedInput, "Remaining bytes in the input")
+
diff --git a/tests/official/test_fixture_ssz_consensus_objects.nim b/tests/official/test_fixture_ssz_consensus_objects.nim
index ecf763752..5bd466c30 100644
--- a/tests/official/test_fixture_ssz_consensus_objects.nim
+++ b/tests/official/test_fixture_ssz_consensus_objects.nim
@@ -41,10 +41,10 @@ setDefaultValue(SSZHashTreeRoot, signing_root, "")
 # Note this only tracks HashTreeRoot
 # Checking the values against the yaml file is TODO (require more flexible Yaml parser)
 
-
 proc checkSSZ(T: type SignedBeaconBlock, dir: string, expectedHash: SSZHashTreeRoot) =
   # Deserialize into a ref object to not fill Nim stack
-  var deserialized = newClone(SSZ.loadFile(dir/"serialized.ssz", T))
+  let encoded = readFileBytes(dir/"serialized.ssz")
+  var deserialized = newClone sszDecodeEntireInput(encoded, T)
 
   # SignedBeaconBlocks usually not hashed because they're identified by
   # htr(BeaconBlock), so do it manually
@@ -52,15 +52,22 @@ proc checkSSZ(T: type SignedBeaconBlock, dir: string, expectedHash: SSZHashTreeR
     [hash_tree_root(deserialized.message),
     hash_tree_root(deserialized.signature)]))
 
-  # TODO check the value
+  check SSZ.encode(deserialized[]) == encoded
+  check sszSize(deserialized[]) == encoded.len
 
-proc checkSSZ(T: typedesc, dir: string, expectedHash: SSZHashTreeRoot) =
+  # TODO check the value (requires YAML loader)
+
+proc checkSSZ(T: type, dir: string, expectedHash: SSZHashTreeRoot) =
   # Deserialize into a ref object to not fill Nim stack
-  var deserialized = newClone(SSZ.loadFile(dir/"serialized.ssz", T))
+  let encoded = readFileBytes(dir/"serialized.ssz")
+  var deserialized = newClone sszDecodeEntireInput(encoded, T)
 
   check: expectedHash.root == "0x" & toLowerASCII($hash_tree_root(deserialized[]))
 
-  # TODO check the value
+  check SSZ.encode(deserialized[]) == encoded
+  check sszSize(deserialized[]) == encoded.len
+
+  # TODO check the value (requires YAML loader)
 
 proc loadExpectedHashTreeRoot(dir: string): SSZHashTreeRoot =
   var s = openFileStream(dir/"roots.yaml")
diff --git a/tests/official/test_fixture_ssz_generic_types.nim b/tests/official/test_fixture_ssz_generic_types.nim
index 087281763..a4226619a 100644
--- a/tests/official/test_fixture_ssz_generic_types.nim
+++ b/tests/official/test_fixture_ssz_generic_types.nim
@@ -33,9 +33,6 @@ type
     # Containers have a root (thankfully) and signing_root field
     signing_root: string
 
-  UnconsumedInput* = object of CatchableError
-  TestSizeError* = object of ValueError
-
 # Make signing root optional
 setDefaultValue(SSZHashTreeRoot, signing_root, "")
 
@@ -79,18 +76,15 @@ type
 proc checkBasic(T: typedesc,
                 dir: string,
                 expectedHash: SSZHashTreeRoot) =
-  var fileContents = readFile(dir/"serialized.ssz")
-  var stream = unsafeMemoryInput(fileContents)
-  var reader = init(SszReader, stream)
-  var deserialized = reader.readValue(T)
-
-  if stream.readable:
-    raise newException(UnconsumedInput, "Remaining bytes in the input")
+  var fileContents = readFileBytes(dir/"serialized.ssz")
+  var deserialized = sszDecodeEntireInput(fileContents, T)
 
   let
     expectedHash = expectedHash.root
     actualHash = "0x" & toLowerASCII($deserialized.hashTreeRoot())
+
   check expectedHash == actualHash
+  check sszSize(deserialized) == fileContents.len
 
   # TODO check the value
 
diff --git a/vendor/nim-serialization b/vendor/nim-serialization
index 1c0e0adf4..7b0d0f364 160000
--- a/vendor/nim-serialization
+++ b/vendor/nim-serialization
@@ -1 +1 @@
-Subproject commit 1c0e0adf459932e45b2a7483f704086cee7c906a
+Subproject commit 7b0d0f364be07b0949d648b6a8b0d3614eb107a8