diff --git a/xarray_beam/_src/core.py b/xarray_beam/_src/core.py
index 3aa80d9..44ca4ee 100644
--- a/xarray_beam/_src/core.py
+++ b/xarray_beam/_src/core.py
@@ -20,6 +20,7 @@
 import itertools
 import math
 import pickle
+import warnings
 import time
 from typing import Any, Generic, TypeVar
 
@@ -272,7 +273,15 @@ def is_deterministic(self) -> bool:
     return False
 
   def estimate_size(self, value: xarray.Dataset) -> int:
-    return value.nbytes
+    nbytes = value.nbytes
+    if nbytes > 2**31 - 1:
+      warnings.warn(
+          f"Dataset size ({nbytes / 2**30:.1f} GB) exceeds Beam's 2 GB "
+          f"counter limit; capping estimate_size to 2^31-1.",
+          stacklevel=2,
+      )
+      return 2**31 - 1
+    return nbytes
 
   def to_type_hint(self) -> type[xarray.Dataset]:
     return xarray.Dataset
diff --git a/xarray_beam/_src/core_test.py b/xarray_beam/_src/core_test.py
index f44f6ac..2e4ddc1 100644
--- a/xarray_beam/_src/core_test.py
+++ b/xarray_beam/_src/core_test.py
@@ -309,6 +309,21 @@ def test_no_fallback_deterministic_coder_warnings(self):
       with beam.Pipeline(runner='DirectRunner') as p:
         p | beam.Create(inputs) | beam.GroupByKey()
 
+  def test_dataset_coder_estimate_size_small(self):
+    dataset = xarray.Dataset({'foo': ('x', np.arange(6))})
+    self.assertEqual(core.DatasetCoder().estimate_size(dataset), dataset.nbytes)
+
+  def test_dataset_coder_estimate_size_overflow(self):
+    # dask-backed array gives us a >2 GB nbytes without allocating memory:
+    # 2**29 * 8 bytes = 2**32 bytes (~4 GB), above Beam's 2**31 - 1 cap.
+    dataset = xarray.Dataset(
+        {'foo': (('x',), da.zeros(2**29, dtype='float64'))}
+    )
+    self.assertGreater(dataset.nbytes, 2**31 - 1)
+    with self.assertWarnsRegex(UserWarning, "exceeds Beam's 2 GB"):
+      size = core.DatasetCoder().estimate_size(dataset)
+    self.assertEqual(size, 2**31 - 1)
+
 
 class DatasetToChunksTest(test_util.TestCase):