diff --git a/dp_wizard/shiny/panels/dataset_panel/__init__.py b/dp_wizard/shiny/panels/dataset_panel/__init__.py
index fcc7b6e8..1654d2c8 100644
--- a/dp_wizard/shiny/panels/dataset_panel/__init__.py
+++ b/dp_wizard/shiny/panels/dataset_panel/__init__.py
@@ -336,7 +336,7 @@ def input_files_upload_ui():
     @render.ui
     def csv_message_ui():
         return data_source.csv_message_ui(
-            csv_column_mismatch_calc=csv_column_mismatch_calc,
+            csv_column_mismatch=csv_column_mismatch_calc(),
             csv_messages=csv_info().get_messages(),
         )
 
diff --git a/dp_wizard/shiny/panels/dataset_panel/data_source.py b/dp_wizard/shiny/panels/dataset_panel/data_source.py
index 222369a9..7421c11f 100644
--- a/dp_wizard/shiny/panels/dataset_panel/data_source.py
+++ b/dp_wizard/shiny/panels/dataset_panel/data_source.py
@@ -37,13 +37,12 @@ def context_code_sample():  # pragma: no cover
 
 
 def csv_message_ui(
-    csv_column_mismatch_calc,
+    csv_column_mismatch,
     csv_messages: list[str],
 ):  # pragma: no cover
     messages = [f"- {m}" for m in csv_messages]
-    mismatch = csv_column_mismatch_calc()
-    if mismatch:
-        just_public, just_private = mismatch
+    if csv_column_mismatch:
+        just_public, just_private = csv_column_mismatch
         if just_public:
             messages.append(
                 "- Only the public data contains: "
diff --git a/dp_wizard/types.py b/dp_wizard/types.py
index 93360e07..11706222 100644
--- a/dp_wizard/types.py
+++ b/dp_wizard/types.py
@@ -114,6 +114,37 @@ def __init__(self, path: Path | None):
             # Only used as initial value
             return
 
+        if not path.exists():
+            self._errors.append(f"No such file: {path}")
+            return
+
+        try:
+            size = path.stat().st_size
+        except Exception:
+            self._errors.append(f"Could not stat file: {path}")
+            return
+        M = 1024 * 1024
+        error_m = 100
+        warn_m = 10
+        assert error_m > warn_m
+        if size > error_m * M:
+            self._errors.append(
+                re.sub(
+                    r"\s+",
+                    " ",
+                    f"""
+                    DP Wizard is an interactive tool, and {size // M}M
+                    would be too slow. DP Wizard is limited to {error_m}M,
+                    although the OpenDP Library itself doesn't have such a limit.
+                    """,
+                ).strip()
+            )
+            return
+        if size > warn_m * M:
+            self._warnings.append(
+                f"Files larger than {warn_m}M may be slow to process."
+            )
+
         try:
             if path.suffix != ".csv":
                 path = convert_to_csv(path)