diff --git a/apps/web/src/components/media/MediaPlayer.test.tsx b/apps/web/src/components/media/MediaPlayer.test.tsx
index 0be2403..c1d9f5a 100644
--- a/apps/web/src/components/media/MediaPlayer.test.tsx
+++ b/apps/web/src/components/media/MediaPlayer.test.tsx
@@ -66,6 +66,21 @@ describe("MediaPlayer", () => {
     expect(video.hasAttribute("controls")).toBe(true);
   });
 
+  it("sizes the frame from the file when the block has no dimensions", async () => {
+    await act(async () =>
+      root.render(<MediaPlayer artifactId="art-4" kind="video" name="up.mp4" />),
+    );
+    const frame = container.querySelector<HTMLDivElement>('[data-testid="media-video"]')!;
+    expect(frame.style.width).toBe("420px");
+    const video = container.querySelector("video")!;
+    Object.defineProperty(video, "videoWidth", { value: 1080 });
+    Object.defineProperty(video, "videoHeight", { value: 1920 });
+    await act(async () => {
+      video.dispatchEvent(new Event("loadedmetadata"));
+    });
+    expect(frame.style.width).toBe("270px");
+  });
+
   it("falls back to a download link when the browser cannot play the file", async () => {
     await act(async () =>
       root.render(<MediaPlayer artifactId="art-3" kind="audio" name="memo.ogg" />),
diff --git a/apps/web/src/components/media/MediaPlayer.tsx b/apps/web/src/components/media/MediaPlayer.tsx
index d793ba4..0df489d 100644
--- a/apps/web/src/components/media/MediaPlayer.tsx
+++ b/apps/web/src/components/media/MediaPlayer.tsx
@@ -28,6 +28,7 @@ export function MediaPlayer({
   const container = useRef<HTMLDivElement>(null);
   const near = useNearViewport(container);
   const [failed, setFailed] = useState(false);
+  const [frame, setFrame] = useState<{ width: number; height: number } | null>(null);
   const src = near ? artifactUrl(artifactId) : undefined;
 
   if (failed) return <MediaFallback artifactId={artifactId} name={name} />;
@@ -48,7 +49,8 @@ export function MediaPlayer({
     );
   }
 
-  const box = videoBox(width, height);
+  // Uploads carry no dimensions; size the frame from the file once its metadata loads.
+  const box = videoBox(width ?? frame?.width, height ?? frame?.height);
   return (
     <div
       ref={container}
@@ -66,6 +68,12 @@ export function MediaPlayer({
         aria-label={name}
         className="block size-full object-contain"
         onError={() => setFailed(true)}
+        onLoadedMetadata={(event) => {
+          const { videoWidth, videoHeight } = event.currentTarget;
+          if (videoWidth > 0 && videoHeight > 0) {
+            setFrame({ width: videoWidth, height: videoHeight });
+          }
+        }}
       />
     </div>
   );