diff --git a/doc/changelog.rst b/doc/changelog.rst
index 0a1fd882..9648c15d 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -29,6 +29,7 @@ General
   - ``sidesite -> MjsSite|None``
   - ``coef -> real``
   - ``divisor -> real``
+- Non-breaking API change: :ref:`mj_sizeModel` now returns an :ref:`mjtSize` rather than an ``int``.
 
 MJX
 ^^^
diff --git a/doc/includes/references.h b/doc/includes/references.h
index ce24d87e..f8eae940 100644
--- a/doc/includes/references.h
+++ b/doc/includes/references.h
@@ -3097,7 +3097,7 @@ mjModel* mj_copyModel(mjModel* dest, const mjModel* src);
 void mj_saveModel(const mjModel* m, const char* filename, void* buffer, int buffer_sz);
 mjModel* mj_loadModel(const char* filename, const mjVFS* vfs);
 void mj_deleteModel(mjModel* m);
-int mj_sizeModel(const mjModel* m);
+mjtSize mj_sizeModel(const mjModel* m);
 mjData* mj_makeData(const mjModel* m);
 mjData* mj_copyData(mjData* dest, const mjModel* m, const mjData* src);
 mjData* mjv_copyData(mjData* dest, const mjModel* m, const mjData* src);
diff --git a/include/mujoco/mujoco.h b/include/mujoco/mujoco.h
index 7100fcaa..5badc217 100644
--- a/include/mujoco/mujoco.h
+++ b/include/mujoco/mujoco.h
@@ -216,7 +216,7 @@ MJAPI mjModel* mj_loadModel(const char* filename, const mjVFS* vfs);
 MJAPI void mj_deleteModel(mjModel* m);
 
 // Return size of buffer needed to hold model.
-MJAPI int mj_sizeModel(const mjModel* m);
+MJAPI mjtSize mj_sizeModel(const mjModel* m);
 
 // Allocate mjData corresponding to given model.
 // If the model buffer is unallocated the initial configuration will not be set.
diff --git a/python/mujoco/introspect/functions.py b/python/mujoco/introspect/functions.py
index 5a8ab934..75aeca47 100644
--- a/python/mujoco/introspect/functions.py
+++ b/python/mujoco/introspect/functions.py
@@ -856,7 +856,7 @@ FUNCTIONS: Mapping[str, FunctionDecl] = dict([
     ('mj_sizeModel',
      FunctionDecl(
          name='mj_sizeModel',
-         return_type=ValueType(name='int'),
+         return_type=ValueType(name='mjtSize'),
          parameters=(
              FunctionParameterDecl(
                  name='m',
diff --git a/src/engine/engine_io.c b/src/engine/engine_io.c
index a2ae5544..32ea64f0 100644
--- a/src/engine/engine_io.c
+++ b/src/engine/engine_io.c
@@ -671,8 +671,8 @@ void mj_deleteModel(mjModel* m) {
 
 
 // size of buffer needed to hold model
-int mj_sizeModel(const mjModel* m) {
-  int size = (
+mjtSize mj_sizeModel(const mjModel* m) {
+  mjtSize size = (
     sizeof(int)*(NHEADER+getnint())
     + sizeof(mjtSize)*getnbuffer()
     + sizeof(mjOption)
diff --git a/src/engine/engine_io.h b/src/engine/engine_io.h
index 7a3df5ac..a2d7bc34 100644
--- a/src/engine/engine_io.h
+++ b/src/engine/engine_io.h
@@ -76,7 +76,7 @@ mjModel* mj_loadModelBuffer(const void* buffer, int buffer_sz);
 MJAPI void mj_deleteModel(mjModel* m);
 
 // size of buffer needed to hold model
-MJAPI int mj_sizeModel(const mjModel* m);
+MJAPI mjtSize mj_sizeModel(const mjModel* m);
 
 // validate reference fields in a model; return null if valid, error message otherwise
 MJAPI const char* mj_validateReferences(const mjModel* m);
diff --git a/unity/Runtime/Bindings/MjBindings.cs b/unity/Runtime/Bindings/MjBindings.cs
index 3239c5f0..b7c3823b 100644
--- a/unity/Runtime/Bindings/MjBindings.cs
+++ b/unity/Runtime/Bindings/MjBindings.cs
@@ -6448,7 +6448,7 @@ public static unsafe extern mjModel_* mj_loadModel([MarshalAs(UnmanagedType.LPSt
 public static unsafe extern void mj_deleteModel(mjModel_* m);
 
 [DllImport("mujoco", CallingConvention = CallingConvention.Cdecl)]
-public static unsafe extern int mj_sizeModel(mjModel_* m);
+public static unsafe extern UInt64 mj_sizeModel(mjModel_* m);
 
 [DllImport("mujoco", CallingConvention = CallingConvention.Cdecl)]
 public static unsafe extern mjData_* mj_makeData(mjModel_* m);
diff --git a/wasm/codegen/generated/bindings.cc b/wasm/codegen/generated/bindings.cc
index 31b5744b..2651bcaf 100644
--- a/wasm/codegen/generated/bindings.cc
+++ b/wasm/codegen/generated/bindings.cc
@@ -10114,7 +10114,7 @@ void mj_defaultVisual_wrapper(MjVisual& vis) {
   mj_defaultVisual(vis.get());
 }
 
-int mj_sizeModel_wrapper(const MjModel& m) {
+mjtSize mj_sizeModel_wrapper(const MjModel& m) {
   return mj_sizeModel(m.get());
 }