diff --git a/news/13395.feature.rst b/news/13395.feature.rst
new file mode 100644
index 00000000000..86dbc0dfb2c
--- /dev/null
+++ b/news/13395.feature.rst
@@ -0,0 +1 @@
+add size attribute to pylock.toml
diff --git a/src/pip/_internal/models/link.py b/src/pip/_internal/models/link.py
index 87651c76e25..2a8567d717d 100644
--- a/src/pip/_internal/models/link.py
+++ b/src/pip/_internal/models/link.py
@@ -508,6 +508,23 @@ def hash_name(self) -> str | None:
     def show_url(self) -> str:
         return posixpath.basename(self._url.split("#", 1)[0].split("?", 1)[0])
 
+    @property
+    def size(self) -> int | None:
+        """Fetch the size of the file from HTTP headers if available."""
+        from pip._internal.network.session import PipSession
+
+        try:
+            session = PipSession()
+            response = session.head(self.url, allow_redirects=True)
+            response.raise_for_status()
+            # Check if the 'Content-Length' header is present
+            size = response.headers.get("Content-Length")
+            if size:
+                return int(size)
+        except ValueError as e:
+            logger.warning("Could not fetch size for %s: %s", self.url, e)
+        return None
+
     @property
     def is_file(self) -> bool:
         return self.scheme == "file"
diff --git a/src/pip/_internal/models/pylock.py b/src/pip/_internal/models/pylock.py
index e7df01a2bc3..474dd76dfa6 100644
--- a/src/pip/_internal/models/pylock.py
+++ b/src/pip/_internal/models/pylock.py
@@ -47,7 +47,7 @@ class PackageDirectory:
 class PackageArchive:
     url: str | None
     # (not supported) path: Optional[str]
-    # (not supported) size: Optional[int]
+    # (not supported) size: int | None
     # (not supported) upload_time: Optional[datetime]
     hashes: dict[str, str]
     subdirectory: str | None
@@ -59,7 +59,7 @@ class PackageSdist:
     # (not supported) upload_time: Optional[datetime]
     url: str | None
     # (not supported) path: Optional[str]
-    # (not supported) size: Optional[int]
+    size: int | None
     hashes: dict[str, str]
 
 
@@ -69,7 +69,7 @@ class PackageWheel:
     # (not supported) upload_time: Optional[datetime]
     url: str | None
     # (not supported) path: Optional[str]
-    # (not supported) size: Optional[int]
+    size: int | None
     hashes: dict[str, str]
 
 
@@ -125,6 +125,7 @@ def from_install_requirement(cls, ireq: InstallRequirement, base_dir: Path) -> S
                     raise NotImplementedError()
                 package.archive = PackageArchive(
                     url=download_info.url,
+                    # size=link.size,
                     hashes=download_info.info.hashes,
                     subdirectory=download_info.subdirectory,
                 )
@@ -142,6 +143,7 @@ def from_install_requirement(cls, ireq: InstallRequirement, base_dir: Path) -> S
                         PackageWheel(
                             name=link.filename,
                             url=download_info.url,
+                            size=link.size,
                             hashes=download_info.info.hashes,
                         )
                     ]
@@ -149,6 +151,7 @@ def from_install_requirement(cls, ireq: InstallRequirement, base_dir: Path) -> S
                     package.sdist = PackageSdist(
                         name=link.filename,
                         url=download_info.url,
+                        size=link.size,
                         hashes=download_info.info.hashes,
                     )
             else:
diff --git a/tests/functional/test_lock.py b/tests/functional/test_lock.py
index 727bd72c8a0..a8da8e38ef9 100644
--- a/tests/functional/test_lock.py
+++ b/tests/functional/test_lock.py
@@ -33,6 +33,7 @@ def test_lock_wheel_from_findlinks(
                 "wheels": [
                     {
                         "name": "simplewheel-2.0-1-py2.py3-none-any.whl",
+                        "size": 2156,
                         "url": path_to_url(
                             str(
                                 shared_data.root
@@ -80,6 +81,7 @@ def test_lock_sdist_from_findlinks(
                     ),
                 },
                 "name": "simple-2.0.tar.gz",
+                "size": 673,
                 "url": path_to_url(
                     str(shared_data.root / "packages" / "simple-2.0.tar.gz")
                 ),
@@ -171,6 +173,7 @@ def test_lock_local_editable_with_dep(
                             / "simplewheel-2.0-1-py2.py3-none-any.whl"
                         )
                     ),
+                    "size": 2156,
                     "hashes": {
                         "sha256": (
                             "71e1ca6b16ae3382a698c284013f6650"
@@ -234,3 +237,17 @@ def test_lock_archive(script: PipTestEnvironment, shared_data: TestData) -> None
             },
         },
     ]
+
+
+def test_lock_includes_size(script: PipTestEnvironment, shared_data: TestData) -> None:
+    script.pip(
+        "lock",
+        "simplewheel==2.0",
+        "--no-index",
+        "--find-links",
+        str(shared_data.root / "packages/"),
+        expect_stderr=True,  # for the experimental warning
+    )
+    pylock = tomllib.loads(script.scratch_path.joinpath("pylock.toml").read_text())
+    assert "size" in pylock["packages"][0]["wheels"][0]
+    assert pylock["packages"][0]["wheels"][0]["size"] > 0
diff --git a/tests/unit/test_link.py b/tests/unit/test_link.py
index 92e92dffa3d..f82f28dc834 100644
--- a/tests/unit/test_link.py
+++ b/tests/unit/test_link.py
@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+from unittest.mock import patch
+
 import pytest
 
 from pip._internal.models.link import Link, links_equivalent
@@ -188,6 +190,12 @@ def test_is_vcs(self, url: str, expected: bool) -> None:
         link = Link(url)
         assert link.is_vcs is expected
 
+    def test_link_size(self) -> None:
+        with patch("pip._internal.network.session.PipSession.head") as mock_head:
+            mock_head.return_value.headers = {"Content-Length": "12345"}
+            link = Link(url="https://example.com/package.tar.gz")
+            assert link.size == 12345
+
 
 @pytest.mark.parametrize(
     "url1, url2",