diff --git a/system/host/fidl/include/fidl/flat_ast.h b/system/host/fidl/include/fidl/flat_ast.h
index 4f53556..b075ea8 100644
--- a/system/host/fidl/include/fidl/flat_ast.h
+++ b/system/host/fidl/include/fidl/flat_ast.h
@@ -58,7 +58,7 @@
     IntType value_;
 };
 
-using Size = IntConstant<uint64_t>;
+using Size = IntConstant<uint32_t>;
 
 // TODO(TO-701) Handle multipart names.
 struct Name {
@@ -123,19 +123,19 @@
         Identifier,
     };
 
-    explicit Type(Kind kind, uint64_t size)
+    explicit Type(Kind kind, uint32_t size)
         : kind(kind), size(size) {}
 
     const Kind kind;
     // Set at construction time for most Types. Identifier types get
     // this set later, during compilation.
-    uint64_t size;
+    uint32_t size;
 
     bool operator<(const Type& other) const;
 };
 
 struct ArrayType : public Type {
-    ArrayType(uint64_t size, std::unique_ptr<Type> element_type, Size element_count)
+    ArrayType(uint32_t size, std::unique_ptr<Type> element_type, Size element_count)
         : Type(Kind::Array, size),
           element_type(std::move(element_type)),
           element_count(std::move(element_count)) {}
@@ -214,7 +214,7 @@
 };
 
 struct PrimitiveType : public Type {
-    static uint64_t SubtypeSize(types::PrimitiveSubtype subtype) {
+    static uint32_t SubtypeSize(types::PrimitiveSubtype subtype) {
         switch (subtype) {
         case types::PrimitiveSubtype::Bool:
         case types::PrimitiveSubtype::Int8:
diff --git a/system/host/fidl/include/fidl/json_generator.h b/system/host/fidl/include/fidl/json_generator.h
index 5d781fb..bf6d979 100644
--- a/system/host/fidl/include/fidl/json_generator.h
+++ b/system/host/fidl/include/fidl/json_generator.h
@@ -63,7 +63,7 @@
     void Generate(bool value);
     void Generate(StringView value);
     void Generate(SourceLocation value);
-    void Generate(uint64_t value);
+    void Generate(uint32_t value);
 
     void Generate(types::HandleSubtype value);
     void Generate(types::Nullability value);
diff --git a/system/host/fidl/include/fidl/type_shape.h b/system/host/fidl/include/fidl/type_shape.h
index 50e9438..f82e23b 100644
--- a/system/host/fidl/include/fidl/type_shape.h
+++ b/system/host/fidl/include/fidl/type_shape.h
@@ -9,7 +9,7 @@
 
 class TypeShape {
 public:
-    constexpr TypeShape(uint64_t size, uint64_t alignment)
+    constexpr TypeShape(uint32_t size, uint32_t alignment)
         : size_(size), alignment_(alignment) {}
     constexpr TypeShape()
         : TypeShape(0u, 0u) {}
@@ -17,17 +17,17 @@
     TypeShape(const TypeShape&) = default;
     TypeShape& operator=(const TypeShape&) = default;
 
-    uint64_t Size() const { return size_; }
-    uint64_t Alignment() const { return alignment_; }
+    uint32_t Size() const { return size_; }
+    uint32_t Alignment() const { return alignment_; }
 
 private:
-    uint64_t size_;
-    uint64_t alignment_;
+    uint32_t size_;
+    uint32_t alignment_;
 };
 
 class FieldShape {
 public:
-    explicit FieldShape(TypeShape typeshape, uint64_t offset = 0u)
+    explicit FieldShape(TypeShape typeshape, uint32_t offset = 0u)
         : typeshape_(typeshape),
           offset_(offset) {}
     FieldShape()
@@ -36,16 +36,16 @@
     TypeShape& Typeshape() { return typeshape_; }
     TypeShape Typeshape() const { return typeshape_; }
 
-    uint64_t Size() const { return typeshape_.Size(); }
-    uint64_t Alignment() const { return typeshape_.Alignment(); }
-    uint64_t Offset() const { return offset_; }
+    uint32_t Size() const { return typeshape_.Size(); }
+    uint32_t Alignment() const { return typeshape_.Alignment(); }
+    uint32_t Offset() const { return offset_; }
 
     void SetTypeshape(TypeShape typeshape) { typeshape_ = typeshape; }
-    void SetOffset(uint64_t offset) { offset_ = offset; }
+    void SetOffset(uint32_t offset) { offset_ = offset; }
 
 private:
     TypeShape typeshape_;
-    uint64_t offset_;
+    uint32_t offset_;
 };
 
 #endif // ZIRCON_SYSTEM_HOST_FIDL_INCLUDE_FIDL_TYPE_SHAPE_H_
diff --git a/system/host/fidl/lib/flat_ast.cpp b/system/host/fidl/lib/flat_ast.cpp
index b741479..7f9995b 100644
--- a/system/host/fidl/lib/flat_ast.cpp
+++ b/system/host/fidl/lib/flat_ast.cpp
@@ -46,7 +46,7 @@
 constexpr TypeShape kFloat64TypeShape = TypeShape(8u, 8u);
 constexpr TypeShape kPointerTypeShape = TypeShape(8u, 8u);
 
-uint64_t AlignTo(uint64_t size, uint64_t alignment) {
+uint32_t AlignTo(uint32_t size, uint32_t alignment) {
     auto mask = alignment - 1;
     size += mask;
     size &= ~mask;
@@ -54,8 +54,8 @@
 }
 
 TypeShape CStructTypeShape(std::vector<FieldShape*>* fields) {
-    uint64_t size = 0u;
-    uint64_t alignment = 1u;
+    uint32_t size = 0u;
+    uint32_t alignment = 1u;
 
     for (FieldShape* field : *fields) {
         field->SetOffset(size);
@@ -69,8 +69,8 @@
 }
 
 TypeShape CUnionTypeShape(const std::vector<flat::Union::Member>& members) {
-    uint64_t size = 0u;
-    uint64_t alignment = 1u;
+    uint32_t size = 0u;
+    uint32_t alignment = 1u;
     for (const auto& member : members) {
         const auto& fieldshape = member.fieldshape;
         size = std::max(size, fieldshape.Size());
@@ -85,7 +85,7 @@
     return CStructTypeShape(fields);
 }
 
-TypeShape ArrayTypeShape(TypeShape element, uint64_t count) {
+TypeShape ArrayTypeShape(TypeShape element, uint32_t count) {
     return TypeShape(element.Size() * count, element.Alignment());
 }
 
@@ -141,7 +141,7 @@
 // so on.
 
 bool Library::ParseSize(std::unique_ptr<raw::Constant> raw_constant, Size* out_size) {
-    uint64_t value;
+    uint32_t value;
     if (!ParseIntegerConstant(raw_constant.get(), &value)) {
         *out_size = Size();
         return false;
@@ -167,7 +167,7 @@
         if (!ParseSize(std::move(array_type->element_count), &element_count))
             return false;
         // TODO(kulakowski) Overflow checking.
-        uint64_t size = element_count.Value() * element_type->size;
+        uint32_t size = element_count.Value() * element_type->size;
         *out_type = std::make_unique<ArrayType>(size, std::move(element_type), std::move(element_count));
         break;
     }
diff --git a/system/host/fidl/lib/json_generator.cpp b/system/host/fidl/lib/json_generator.cpp
index d7960f3..a77deca 100644
--- a/system/host/fidl/lib/json_generator.cpp
+++ b/system/host/fidl/lib/json_generator.cpp
@@ -50,10 +50,6 @@
     *file << value;
 }
 
-void EmitUint64(std::ostream* file, uint64_t value) {
-    *file << value;
-}
-
 void EmitNewline(std::ostream* file) {
     *file << "\n";
 }
@@ -174,8 +170,8 @@
     EmitString(&json_file_, value.data());
 }
 
-void JSONGenerator::Generate(uint64_t value) {
-    EmitUint64(&json_file_, value);
+void JSONGenerator::Generate(uint32_t value) {
+    EmitUint32(&json_file_, value);
 }
 
 void JSONGenerator::Generate(types::HandleSubtype value) {
diff --git a/system/host/fidl/lib/tables_generator.cpp b/system/host/fidl/lib/tables_generator.cpp
index 9a6f4c7..9bdcb73 100644
--- a/system/host/fidl/lib/tables_generator.cpp
+++ b/system/host/fidl/lib/tables_generator.cpp
@@ -272,8 +272,8 @@
         if (iter != vector_type_map_.end())
             return iter->second;
         auto coded_element_type = Compile(vector_type->element_type.get());
-        uint64_t max_count = vector_type->element_count.Value();
-        uint64_t element_size = vector_type->element_type->size;
+        uint32_t max_count = vector_type->element_count.Value();
+        uint32_t element_size = vector_type->element_type->size;
         StringView element_name = coded_element_type->coded_name;
         auto name = NameCodedVector(element_name, max_count, vector_type->nullability);
         auto coded_vector_type = std::make_unique<coded::VectorType>(std::move(name), coded_element_type, max_count, element_size, vector_type->nullability);