diff --git a/tests/entrypoints/openai_api/test_image_server.py b/tests/entrypoints/openai_api/test_image_server.py
index d108b9323..bf3d8bd61 100644
--- a/tests/entrypoints/openai_api/test_image_server.py
+++ b/tests/entrypoints/openai_api/test_image_server.py
@@ -11,10 +11,11 @@ import base64
 import io
 import json
 from argparse import Namespace
+from http import HTTPStatus
 from types import SimpleNamespace
 
 import pytest
-from fastapi import FastAPI
+from fastapi import FastAPI, HTTPException
 from fastapi.testclient import TestClient
 from PIL import Image
 from pytest_mock import MockerFixture
@@ -23,7 +24,7 @@ from vllm.entrypoints.openai.models.protocol import BaseModelPath
 from vllm.sampling_params import RequestOutputKind
 
 from vllm_omni.entrypoints.async_omni import AsyncOmni
-from vllm_omni.entrypoints.openai.api_server import _DiffusionServingModels, router
+from vllm_omni.entrypoints.openai.api_server import _check_max_generated_image_size, _DiffusionServingModels, router
 from vllm_omni.entrypoints.openai.image_api_utils import (
     encode_image_base64,
     parse_size,
@@ -1979,6 +1980,56 @@ def test_extract_images_from_result():
     assert images[0].size == (32, 32)
 
 
+# ---------------------------------------------------------------------------
+# _check_max_generated_image_size unit tests
+# ---------------------------------------------------------------------------
+
+
+def test_width_height_within_limit_passes():
+    args = SimpleNamespace(max_generated_image_size=1024 * 1024)
+    # Exactly at limit is allowed (> not >=)
+    _check_max_generated_image_size(args, 1024, 1024)
+    # Below limit
+    _check_max_generated_image_size(args, 512, 512)
+
+
+def test_width_height_exceeds_limit_raises_400():
+    limit = 1024 * 1024
+    args = SimpleNamespace(max_generated_image_size=limit)
+    with pytest.raises(HTTPException) as exc_info:
+        _check_max_generated_image_size(args, 1025, 1024)
+    assert exc_info.value.status_code == HTTPStatus.BAD_REQUEST.value
+    assert "1025x1024" in exc_info.value.detail
+    assert str(limit) in exc_info.value.detail
+
+
+def test_width_height_error_message_contains_size_hint():
+    args = SimpleNamespace(max_generated_image_size=512 * 512)
+    with pytest.raises(HTTPException) as exc_info:
+        _check_max_generated_image_size(args, 1024, 512)
+    assert "--max-generated-image-size" in exc_info.value.detail
+
+
+def test_resolution_within_limit_passes():
+    args = SimpleNamespace(max_generated_image_size=1024 * 1024)
+    # Exactly at limit is allowed (> not >=): 1024*1024 == limit
+    _check_max_generated_image_size(args, None, None, resolution=1024)
+    # Below limit
+    _check_max_generated_image_size(args, None, None, resolution=512)
+
+
+def test_resolution_exceeds_limit_raises_400():
+    limit = 1024 * 1024
+    args = SimpleNamespace(max_generated_image_size=limit)
+    with pytest.raises(HTTPException) as exc_info:
+        _check_max_generated_image_size(args, None, None, resolution=1025)
+    assert exc_info.value.status_code == HTTPStatus.BAD_REQUEST.value
+    detail = exc_info.value.detail
+    assert "1025" in detail
+    assert "1025x1025" in detail
+    assert str(limit) in detail
+
+
 def test_image_edits_size_auto_preserves_bridge_size(async_omni_stage_configs_only_client):
     """size=auto must NOT pin the diffusion stage sampling_params.height/width.
 
diff --git a/vllm_omni/entrypoints/cli/serve.py b/vllm_omni/entrypoints/cli/serve.py
index 5b6c7d9b3..9468a632f 100644
--- a/vllm_omni/entrypoints/cli/serve.py
+++ b/vllm_omni/entrypoints/cli/serve.py
@@ -638,8 +638,9 @@ class OmniServeCommand(CLISubcommand):
         # Diffusion model mixed precision
         omni_config_group.add_argument(
             "--max-generated-image-size",
+            default=7680 * 4320,  # 8K resolution
             type=int,
-            help="The max size of generate image (height * width).",
+            help="Maximum generated image size in pixels (height * width).",
         )
 
         # TTS-specific parameters
diff --git a/vllm_omni/entrypoints/openai/api_server.py b/vllm_omni/entrypoints/openai/api_server.py
index 36c8201e0..f70a439f5 100755
--- a/vllm_omni/entrypoints/openai/api_server.py
+++ b/vllm_omni/entrypoints/openai/api_server.py
@@ -2333,7 +2333,8 @@ def _check_max_generated_image_size(
             raise HTTPException(
                 status_code=HTTPStatus.BAD_REQUEST.value,
                 detail=f"Requested image size {width}x{height} exceeds the maximum allowed "
-                f"size of {max_generated_image_size} pixels.",
+                f"size of {max_generated_image_size} pixels. You can reduce the requested size "
+                f"or increase the server's --max-generated-image-size limit.",
             )
     elif resolution is not None:
         # When resolution is set, the output size is resolution * resolution
@@ -2341,7 +2342,8 @@ def _check_max_generated_image_size(
             raise HTTPException(
                 status_code=HTTPStatus.BAD_REQUEST.value,
                 detail=f"Requested resolution {resolution} (max {resolution}x{resolution} pixels) "
-                f"exceeds the maximum allowed size of {max_generated_image_size} pixels.",
+                f"exceeds the maximum allowed size of {max_generated_image_size} pixels. "
+                f"You can reduce the requested size or increase the server's --max-generated-image-size limit.",
             )