diff --git a/pennylane/concurrency/executors/external/mpi.py b/pennylane/concurrency/executors/external/mpi.py
index 5b36e059d3b..67fff5191f5 100644
--- a/pennylane/concurrency/executors/external/mpi.py
+++ b/pennylane/concurrency/executors/external/mpi.py
@@ -56,7 +56,7 @@ def __init__(self, max_workers: Optional[int] = None, persist: bool = False, **k
         # Handle set-up upon object creation only.
         from mpi4py import MPI
 
-        self._size = max_workers
+        self._size = max_workers if max_workers else MPI.COMM_WORLD.Get_size() - 1
         self._comm = MPI.COMM_WORLD
 
         self._cfg = ExecBackendConfig(
@@ -84,10 +84,6 @@ def __call__(self, dispatch: str, fn: Callable, *args, **kwargs):
             **kwargs,
         )
 
-    @property
-    def size(self):
-        return self._size
-
     def submit(self, fn: Callable, *args, **kwargs):
         with self._exec_backend()(max_workers=self.size, use_pkl5=True) as executor:
             output_f = executor.submit(fn, *args, **kwargs)
@@ -194,10 +190,6 @@ def starmap(self, fn: Callable, args: Sequence[tuple], **kwargs):
             output_f = executor.starmap(fn, args, chunksize=chunksize, **kwargs)
         return list(output_f)
 
-    @property
-    def size(self):
-        return self._size
-
     def shutdown(self):
         "Shutdown the executor backend, if valid."