ENH: Report how far off the formatting is (#57667)

Author: larsoner

pandas/io/excel/_base.py

@@ -1327,7 +1327,8 @@ def _value_with_fmt(
             # xref https://support.microsoft.com/en-au/office/excel-specifications-and-limits-1672b34d-7043-467e-8e27-269d656771c3
             if len(val) > 32767:
                 warnings.warn(
-                    "Cell contents too long, truncated to 32767 characters",
+                    f"Cell contents too long ({len(val)}), "
+                    "truncated to 32767 characters",
                     UserWarning,
                     stacklevel=find_stack_level(),
                 )

pandas/tests/io/excel/test_writers.py

@@ -1390,7 +1390,7 @@ def test_to_excel_empty_frame(self, engine, tmp_excel):
     def test_to_excel_raising_warning_when_cell_character_exceed_limit(self):
         # GH#56954
         df = DataFrame({"A": ["a" * 32768]})
-        msg = "Cell contents too long, truncated to 32767 characters"
+        msg = r"Cell contents too long \(32768\), truncated to 32767 characters"
         with tm.assert_produces_warning(
             UserWarning, match=msg, raise_on_extra_warnings=False
         ):