Use maximum frame size in IHDR chunk when saving

Tests/test_file_apng.py

@@ -668,6 +668,16 @@ def test_apng_save_blend(tmp_path: Path) -> None:
         assert im.getpixel((0, 0)) == (0, 255, 0, 255)
 
 
+def test_apng_save_size(tmp_path: Path) -> None:
+    test_file = str(tmp_path / "temp.png")
+
+    im = Image.new("L", (100, 100))
+    im.save(test_file, save_all=True, append_images=[Image.new("L", (200, 200))])
+
+    with Image.open(test_file) as reloaded:
+        assert reloaded.size == (200, 200)
+
+
 def test_seek_after_close() -> None:
     im = Image.open("Tests/images/apng/delay.png")
     im.seek(1)

src/PIL/PngImagePlugin.py

@@ -1232,16 +1232,20 @@ def _save(im, fp, filename, chunk=putchunk, save_all=False):
             "default_image", im.info.get("default_image")
         )
         modes = set()
+        sizes = set()
         append_images = im.encoderinfo.get("append_images", [])
         for im_seq in itertools.chain([im], append_images):
             for im_frame in ImageSequence.Iterator(im_seq):
                 modes.add(im_frame.mode)
+                sizes.add(im_frame.size)
         for mode in ("RGBA", "RGB", "P"):
             if mode in modes:
                 break
         else:
             mode = modes.pop()
+        size = tuple(max(frame_size[i] for frame_size in sizes) for i in range(2))
     else:
+        size = im.size
         mode = im.mode
 
     if mode == "P":
@@ -1289,8 +1293,8 @@ def _save(im, fp, filename, chunk=putchunk, save_all=False):
     chunk(
         fp,
         b"IHDR",
-        o32(im.size[0]),  # 0: size
+        o32(size[0]),  # 0: size
-        o32(im.size[1]),
+        o32(size[1]),
         mode,  # 8: depth/type
         b"\0",  # 10: compression
         b"\0",  # 11: filter category