diff --git a/docs/builtin-tools.md b/docs/builtin-tools.md
index 85acb7dd37..02e5a5ccc0 100644
--- a/docs/builtin-tools.md
+++ b/docs/builtin-tools.md
@@ -353,6 +353,23 @@ assert isinstance(result.output, BinaryImage)
 
 _(This example is complete, it can be run "as is")_
 
+To control the image resolution with Google image generation models (Gemini 3 Pro Image and later), use the `size` parameter:
+
+```py {title="image_generation_google_resolution.py"}
+from pydantic_ai import Agent, BinaryImage, ImageGenerationTool
+
+agent = Agent(
+    'google-gla:gemini-3-pro-image-preview',
+    builtin_tools=[ImageGenerationTool(aspect_ratio='16:9', size='4K')],
+    output_type=BinaryImage,
+)
+
+result = agent.run_sync('Generate a high-resolution wide landscape illustration of an axolotl.')
+assert isinstance(result.output, BinaryImage)
+```
+
+_(This example is complete, it can be run "as is")_
+
 For more details, check the [API documentation][pydantic_ai.builtin_tools.ImageGenerationTool].
 
 #### Provider Support
@@ -366,8 +383,8 @@ For more details, check the [API documentation][pydantic_ai.builtin_tools.ImageG
 | `output_format` | ✅ | ❌ |
 | `partial_images` | ✅ | ❌ |
 | `quality` | ✅ | ❌ |
-| `size` | ✅ | ❌ |
-| `aspect_ratio` | ✅ (1:1, 2:3, 3:2) | ✅ |
+| `size` | ✅ (1024x1024, 1024x1536, 1536x1024, auto) | ✅ (1K, 2K, 4K) |
+| `aspect_ratio` | ✅ (1:1, 2:3, 3:2) | ✅ (1:1, 2:3, 3:2, 3:4, 4:3, 4:5, 5:4, 9:16, 16:9, 21:9) |
 
 ## Web Fetch Tool
 
diff --git a/pydantic_ai_slim/pydantic_ai/builtin_tools.py b/pydantic_ai_slim/pydantic_ai/builtin_tools.py
index 912538b715..7fef44eaeb 100644
--- a/pydantic_ai_slim/pydantic_ai/builtin_tools.py
+++ b/pydantic_ai_slim/pydantic_ai/builtin_tools.py
@@ -311,12 +311,13 @@ class ImageGenerationTool(AbstractBuiltinTool):
     * OpenAI Responses
     """
 
-    size: Literal['1024x1024', '1024x1536', '1536x1024', 'auto'] = 'auto'
+    size: Literal['1024x1024', '1024x1536', '1536x1024', '1K', '2K', '4K', 'auto'] = 'auto'
     """The size of the generated image.
 
     Supported by:
 
-    * OpenAI Responses
+    * OpenAI Responses: '1024x1024', '1024x1536', '1536x1024', 'auto'
+    * Google (Gemini 3 Pro Image and later): '1K', '2K', '4K'
     """
 
     aspect_ratio: ImageAspectRatio | None = None
diff --git a/pydantic_ai_slim/pydantic_ai/models/google.py b/pydantic_ai_slim/pydantic_ai/models/google.py
index c6f5459f08..2cde4c5fb5 100644
--- a/pydantic_ai_slim/pydantic_ai/models/google.py
+++ b/pydantic_ai_slim/pydantic_ai/models/google.py
@@ -362,8 +362,12 @@ def _get_tools(
                         raise UserError(
                             "`ImageGenerationTool` is not supported by this model. Use a model with 'image' in the name instead."
                         )
-                    if tool.aspect_ratio:
-                        image_config = ImageConfigDict(aspect_ratio=tool.aspect_ratio)
+
+                    image_config = ImageConfigDict()
+                    if tool.aspect_ratio is not None:
+                        image_config['aspect_ratio'] = tool.aspect_ratio
+                    if tool.size in ('1K', '2K', '4K'):
+                        image_config['image_size'] = tool.size
                 else:  # pragma: no cover
                     raise UserError(
                         f'`{tool.__class__.__name__}` is not supported by `GoogleModel`. If it should be, please file an issue.'
diff --git a/pydantic_ai_slim/pydantic_ai/models/openai.py b/pydantic_ai_slim/pydantic_ai/models/openai.py
index efe9629c3a..67935592d8 100644
--- a/pydantic_ai_slim/pydantic_ai/models/openai.py
+++ b/pydantic_ai_slim/pydantic_ai/models/openai.py
@@ -158,7 +158,11 @@ def _resolve_openai_image_generation_size(
     """Map `ImageGenerationTool.aspect_ratio` to an OpenAI size string when provided."""
     aspect_ratio = tool.aspect_ratio
     if aspect_ratio is None:
-        return tool.size
+        # Only return size if it's an OpenAI-supported value
+        if tool.size in ('auto', '1024x1024', '1024x1536', '1536x1024'):
+            return tool.size
+        # Default to auto if an unsupported size is provided
+        return 'auto'
 
     mapped_size = _OPENAI_ASPECT_RATIO_TO_SIZE.get(aspect_ratio)
     if mapped_size is None:
diff --git a/tests/models/test_google.py b/tests/models/test_google.py
index be6d4bd68a..5833b8ee2b 100644
--- a/tests/models/test_google.py
+++ b/tests/models/test_google.py
@@ -3652,6 +3652,26 @@ async def test_google_image_generation_tool_aspect_ratio(google_provider: Google
     assert image_config == {'aspect_ratio': '16:9'}
 
 
+async def test_google_image_generation_resolution(google_provider: GoogleProvider) -> None:
+    """Test that resolution parameter from ImageGenerationTool is added to image_config."""
+    model = GoogleModel('gemini-3-pro-image-preview', provider=google_provider)
+    params = ModelRequestParameters(builtin_tools=[ImageGenerationTool(size='2K')])
+
+    tools, image_config = model._get_tools(params)  # pyright: ignore[reportPrivateUsage]
+    assert tools is None
+    assert image_config == {'image_size': '2K'}
+
+
+async def test_google_image_generation_resolution_with_aspect_ratio(google_provider: GoogleProvider) -> None:
+    """Test that resolution and aspect_ratio from ImageGenerationTool work together."""
+    model = GoogleModel('gemini-3-pro-image-preview', provider=google_provider)
+    params = ModelRequestParameters(builtin_tools=[ImageGenerationTool(aspect_ratio='16:9', size='4K')])
+
+    tools, image_config = model._get_tools(params)  # pyright: ignore[reportPrivateUsage]
+    assert tools is None
+    assert image_config == {'aspect_ratio': '16:9', 'image_size': '4K'}
+
+
 async def test_google_vertexai_image_generation(allow_model_requests: None, vertex_provider: GoogleProvider):
     model = GoogleModel('gemini-2.5-flash-image', provider=vertex_provider)
 
diff --git a/tests/models/test_openai_responses.py b/tests/models/test_openai_responses.py
index b03e99bb91..092e2be49f 100644
--- a/tests/models/test_openai_responses.py
+++ b/tests/models/test_openai_responses.py
@@ -162,6 +162,11 @@ def test_openai_responses_image_generation_tool_aspect_ratio_conflicts_with_size
         _resolve_openai_image_generation_size(tool)
 
 
+def test_openai_responses_image_generation_tool_unsupported_size_falls_back_to_auto() -> None:
+    tool = ImageGenerationTool(size='2K')
+    assert _resolve_openai_image_generation_size(tool) == 'auto'
+
+
 async def test_openai_responses_model_simple_response_with_tool_call(allow_model_requests: None, openai_api_key: str):
     model = OpenAIResponsesModel('gpt-4o', provider=OpenAIProvider(api_key=openai_api_key))
 
diff --git a/tests/test_examples.py b/tests/test_examples.py
index 8ed0828250..bc887504d2 100644
--- a/tests/test_examples.py
+++ b/tests/test_examples.py
@@ -698,6 +698,12 @@ async def model_logic(  # noqa: C901
                     FilePart(content=BinaryImage(data=b'fake', media_type='image/png', identifier='wide-axolotl-city')),
                 ]
             )
+        elif m.content == 'Generate a high-resolution wide landscape illustration of an axolotl.':
+            return ModelResponse(
+                parts=[
+                    FilePart(content=BinaryImage(data=b'fake', media_type='image/png', identifier='high-res-axolotl')),
+                ]
+            )
         elif m.content == 'Generate a chart of y=x^2 for x=-5 to 5.':
             return ModelResponse(
                 parts=[