diff --git a/src/libexpr/attr-set.hh b/src/libexpr/attr-set.hh
index cb29ee74e..0385edbc1 100644
--- a/src/libexpr/attr-set.hh
+++ b/src/libexpr/attr-set.hh
@@ -93,12 +93,12 @@ public:
         return res;
     }
 
-    Size words() const
+    size_t words() const
     {
         return wordsFor(capacity());
     }
 
-    static Size wordsFor(size_t capacity)
+    static size_t wordsFor(size_t capacity)
     {
         return 2 + 3 * capacity; // FIXME
     }
diff --git a/src/libexpr/eval.hh b/src/libexpr/eval.hh
index 4ed8fc541..36bef09e3 100644
--- a/src/libexpr/eval.hh
+++ b/src/libexpr/eval.hh
@@ -70,12 +70,12 @@ public:
         return getMisc() & 0xffff;
     }
 
-    Size words() const
+    size_t words() const
     {
         return wordsFor(getSize());
     }
 
-    static Size wordsFor(unsigned short size)
+    static size_t wordsFor(unsigned short size)
     {
         return 2 + size;
     }
diff --git a/src/libexpr/gc.cc b/src/libexpr/gc.cc
index 96b2728bf..58107690d 100644
--- a/src/libexpr/gc.cc
+++ b/src/libexpr/gc.cc
@@ -239,8 +239,8 @@ void GC::gc()
 
     // Go through all the arenas and add free objects to the
     // appropriate freelists.
-    Size totalObjectsFreed = 0;
-    Size totalWordsFreed = 0;
+    size_t totalObjectsFreed = 0;
+    size_t totalWordsFreed = 0;
 
     for (auto & arena : arenas) {
         auto [objectsFreed, wordsFreed] = freeUnmarked(arena);
@@ -252,10 +252,10 @@ void GC::gc()
         totalWordsFreed * WORD_SIZE, totalObjectsFreed, marked);
 }
 
-std::pair<Size, Size> GC::freeUnmarked(Arena & arena)
+std::pair<size_t, size_t> GC::freeUnmarked(Arena & arena)
 {
-    Size objectsFreed = 0;
-    Size wordsFreed = 0;
+    size_t objectsFreed = 0;
+    size_t wordsFreed = 0;
 
     auto end = arena.start + arena.size;
     auto pos = arena.start;
@@ -272,7 +272,7 @@ std::pair<Size, Size> GC::freeUnmarked(Arena & arena)
         auto obj = (Object *) pos;
         auto tag = obj->type;
 
-        Size objSize;
+        size_t objSize;
         if (tag >= tInt && tag <= tFloat) {
             objSize = ((Value *) obj)->words();
         } else {
@@ -326,7 +326,7 @@ std::pair<Size, Size> GC::freeUnmarked(Arena & arena)
             } else {
                 //debug("FREE OBJECT %x %d %d", obj, obj->type, objSize);
                 #if GC_DEBUG
-                for (Size i = 0; i < objSize; ++i)
+                for (size_t i = 0; i < objSize; ++i)
                     ((Word *) obj)[i] = 0xdeadc0dedeadbeefULL;
                 #endif
                 objectsFreed += 1;
diff --git a/src/libexpr/gc.hh b/src/libexpr/gc.hh
index 0bc951923..f8be832eb 100644
--- a/src/libexpr/gc.hh
+++ b/src/libexpr/gc.hh
@@ -11,7 +11,6 @@
 namespace nix {
 
 typedef unsigned long Word;
-typedef size_t Size; // size in words
 
 enum Tag {
     tFree = 3,
@@ -103,27 +102,27 @@ struct PtrList : Object
 {
     T * elems[0];
 
-    PtrList(Tag type, Size size) : Object(type, size)
+    PtrList(Tag type, size_t size) : Object(type, size)
     {
-        for (Size i = 0; i < size; i++)
+        for (size_t i = 0; i < size; i++)
             elems[i] = nullptr;
     }
 
-    Size size() const { return getMisc(); }
-    Size words() const { return wordsFor(size()); }
-    static Size wordsFor(Size size) { return 1 + size; }
+    size_t size() const { return getMisc(); }
+    size_t words() const { return wordsFor(size()); }
+    static size_t wordsFor(size_t size) { return 1 + size; }
 };
 
 struct Free : Object
 {
     Free * next;
 
-    Free(Size size) : Object(tFree, size), next(nullptr) { }
+    Free(size_t size) : Object(tFree, size), next(nullptr) { }
 
     // return size in words
-    Size words() const { return getMisc(); }
+    size_t words() const { return getMisc(); }
 
-    void setSize(Size size) { assert(size >= 1); setMisc(size); }
+    void setSize(size_t size) { assert(size >= 1); setMisc(size); }
 };
 
 template<class T>
@@ -151,10 +150,10 @@ private:
 
     struct Arena
     {
-        Size size; // in words
+        size_t size; // in words
         Word * start;
 
-        Arena(Size size)
+        Arena(size_t size)
             : size(size)
             , start(new Word[size])
         {
@@ -183,13 +182,13 @@ private:
 
     struct FreeList
     {
-        Size minSize;
+        size_t minSize;
         Free * front = nullptr;
     };
 
     std::array<FreeList, 8> freeLists;
 
-    Object * allocObject(Size size)
+    Object * allocObject(size_t size)
     {
         assert(size >= 2);
 
@@ -253,7 +252,7 @@ public:
     ~GC();
 
     template<typename T, typename... Args>
-    Ptr<T> alloc(Size size, const Args & ... args)
+    Ptr<T> alloc(size_t size, const Args & ... args)
     {
         auto raw = allocObject(size);
         return new (raw) T(args...);
@@ -279,7 +278,7 @@ private:
 
     void addToFreeList(Free * obj);
 
-    std::pair<Size, Size> freeUnmarked(Arena & arena);
+    std::pair<size_t, size_t> freeUnmarked(Arena & arena);
 };
 
 extern GC gc;
diff --git a/src/libexpr/value.hh b/src/libexpr/value.hh
index 2289b86fd..1112511fd 100644
--- a/src/libexpr/value.hh
+++ b/src/libexpr/value.hh
@@ -169,7 +169,7 @@ public:
         return type == tList0 ? 0 : type == tList1 ? 1 : type == tList2 ? 2 : bigList->size();
     }
 
-    constexpr static Size words() { return 3; } // FIXME
+    constexpr static size_t words() { return 3; } // FIXME
 
     void setContext(const PathSet & context)
     {