diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,11 @@
 # ChangeLog
 
+## 0.5.1.0
+
+* Update to TH to support sum types with more than 62 constructors.
+
+* Uses TH to derive Either instance, so that it can sometimes have ConstSize #119.
+
 ## 0.5.0.1
 
 * Updates to test-suite enabling `store` to build with newer dependencies.
diff --git a/src/Data/Store/Internal.hs b/src/Data/Store/Internal.hs
--- a/src/Data/Store/Internal.hs
+++ b/src/Data/Store/Internal.hs
@@ -688,7 +688,6 @@
 instance Store a => Store (First a)
 instance Store a => Store (Last a)
 instance Store a => Store (Maybe a)
-instance (Store a, Store b) => Store (Either a b)
 
 -- FIXME: have TH deriving handle unboxed fields?
 
@@ -702,6 +701,7 @@
     instance Deriving (Store Any)
     instance Deriving (Store Void)
     instance Deriving (Store Bool)
+    instance (Store a, Store b) => Deriving (Store (Either a b))
     |]))
 
 -- TODO: higher arities?  Limited now by Generics instances for tuples
diff --git a/src/Data/Store/TH/Internal.hs b/src/Data/Store/TH/Internal.hs
--- a/src/Data/Store/TH/Internal.hs
+++ b/src/Data/Store/TH/Internal.hs
@@ -103,9 +103,16 @@
     sizeNames = zipWith (\_ -> mkName . ("sz" ++) . show) cons ints
     tagName = mkName "tag"
     valName = mkName "val"
-    sizeExpr =
-        caseE (tupE (concatMap (map sizeAtType . snd) cons))
-              (if null sizeNames then [matchConstSize] else [matchConstSize, matchVarSize])
+    sizeExpr
+        -- Maximum size of GHC tuples
+        | length cons <= 62 =
+            caseE (tupE (concatMap (map sizeAtType . snd) cons))
+                  (case cons of
+                     -- Avoid overlapping matches when the case expression is ()
+                     [] -> [matchConstSize]
+                     [c] | null (snd c) -> [matchConstSize]
+                     _ -> [matchConstSize, matchVarSize])
+        | otherwise = varSizeExpr
       where
         sizeAtType :: (Name, Type) -> ExpQ
         sizeAtType (_, ty) = [| size :: Size $(return ty) |]
@@ -139,9 +146,11 @@
         matchVarSize :: MatchQ
         matchVarSize = do
             match (tupP (map (\(n, _) -> varP n) (concatMap snd cons)))
-                  (normalB [| VarSize $ \x -> tagSize +
-                                  $(caseE [| x |] (map matchVar cons)) |])
+                  (normalB varSizeExpr)
                   []
+        varSizeExpr :: ExpQ
+        varSizeExpr =
+            [| VarSize $ \x -> tagSize + $(caseE [| x |] (map matchVar cons)) |]
         matchVar :: (Name, [(Name, Type)]) -> MatchQ
         matchVar (cname, []) =
             match (conP cname []) (normalB [| 0 |]) []
diff --git a/store.cabal b/store.cabal
--- a/store.cabal
+++ b/store.cabal
@@ -1,13 +1,13 @@
-cabal-version: >= 1.10
+cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.29.0.
+-- This file has been generated from package.yaml by hpack version 0.31.1.
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: cf65d77ec7461cd466e325a485e2ac48b5cbfcf303c243e11f5b8e9674d9057e
+-- hash: 5c038df3338fc7661664ddc082dde9288b48c1153a7680a63b2aed528590535e
 
 name:           store
-version:        0.5.0.1
+version:        0.5.1.0
 synopsis:       Fast binary serialization
 category:       Serialization, Data
 homepage:       https://github.com/fpco/store#readme
@@ -151,12 +151,14 @@
     , void >=0.5.11
   default-language: Haskell2010
 
-test-suite store-weigh
+benchmark store-bench
   type: exitcode-stdio-1.0
-  main-is: Allocations.hs
+  main-is: Bench.hs
+  other-modules:
+      Paths_store
   hs-source-dirs:
-      test
-  ghc-options: -Wall -fwarn-tabs -fwarn-incomplete-uni-patterns -fwarn-incomplete-record-updates -O2 -threaded -rtsopts -with-rtsopts=-N -with-rtsopts=-T -O2
+      bench
+  ghc-options: -Wall -fwarn-tabs -fwarn-incomplete-uni-patterns -fwarn-incomplete-record-updates -O2 -threaded -rtsopts -with-rtsopts=-N1 -with-rtsopts=-s -with-rtsopts=-qg
   build-depends:
       array >=0.5.0.0
     , async >=2.0.2
@@ -165,8 +167,6 @@
     , base64-bytestring >=0.1.1
     , bifunctors >=4.0
     , bytestring >=0.10.4.0
-    , cereal
-    , cereal-vector
     , containers >=0.5.5.1
     , contravariant >=1.3
     , criterion
@@ -203,19 +203,24 @@
     , transformers >=0.3.0.0
     , unordered-containers >=0.2.5.1
     , vector >=0.10.12.3
-    , vector-binary-instances
     , void >=0.5.11
-    , weigh
+  if flag(comparison-bench)
+    cpp-options: -DCOMPARISON_BENCH
+    build-depends:
+        binary
+      , cereal
+      , cereal-vector
+      , vector-binary-instances
+  if flag(small-bench)
+    cpp-options: -DSMALL_BENCH
   default-language: Haskell2010
 
-benchmark store-bench
+benchmark store-weigh
   type: exitcode-stdio-1.0
-  main-is: Bench.hs
-  other-modules:
-      Paths_store
+  main-is: Allocations.hs
   hs-source-dirs:
-      bench
-  ghc-options: -Wall -fwarn-tabs -fwarn-incomplete-uni-patterns -fwarn-incomplete-record-updates -O2 -threaded -rtsopts -with-rtsopts=-N1 -with-rtsopts=-s -with-rtsopts=-qg
+      test
+  ghc-options: -Wall -fwarn-tabs -fwarn-incomplete-uni-patterns -fwarn-incomplete-record-updates -O2 -threaded -rtsopts -with-rtsopts=-N -with-rtsopts=-T -O2
   build-depends:
       array >=0.5.0.0
     , async >=2.0.2
@@ -224,6 +229,8 @@
     , base64-bytestring >=0.1.1
     , bifunctors >=4.0
     , bytestring >=0.10.4.0
+    , cereal
+    , cereal-vector
     , containers >=0.5.5.1
     , contravariant >=1.3
     , criterion
@@ -260,14 +267,7 @@
     , transformers >=0.3.0.0
     , unordered-containers >=0.2.5.1
     , vector >=0.10.12.3
+    , vector-binary-instances
     , void >=0.5.11
-  if flag(comparison-bench)
-    cpp-options: -DCOMPARISON_BENCH
-    build-depends:
-        binary
-      , cereal
-      , cereal-vector
-      , vector-binary-instances
-  if flag(small-bench)
-    cpp-options: -DSMALL_BENCH
+    , weigh
   default-language: Haskell2010