diff --git a/src/textual/css/_style_properties.py b/src/textual/css/_style_properties.py
index 12a76b202..8be05f045 100644
--- a/src/textual/css/_style_properties.py
+++ b/src/textual/css/_style_properties.py
@@ -115,12 +115,12 @@ class ScalarProperty:
         ):
             raise StyleValueError("'auto' not allowed here")
 
-        if new_value.unit != Unit.AUTO:
-            if new_value is not None and new_value.unit not in self.units:
+        if new_value is not None and new_value.unit != Unit.AUTO:
+            if new_value.unit not in self.units:
                 raise StyleValueError(
                     f"{self.name} units must be one of {friendly_list(get_symbols(self.units))}"
                 )
-            if new_value is not None and new_value.is_percent:
+            if new_value.is_percent:
                 new_value = Scalar(
                     float(new_value.value), self.percent_unit, Unit.WIDTH
                 )
diff --git a/tests/css/test_styles.py b/tests/css/test_styles.py
index 039d5745f..49e4545ad 100644
--- a/tests/css/test_styles.py
+++ b/tests/css/test_styles.py
@@ -1,11 +1,15 @@
+from decimal import Decimal
+
 import pytest
 
 from rich.style import Style
 
 from textual.color import Color
-from textual.css.errors import StyleTypeError
+from textual.css.errors import StyleTypeError, StyleValueError
+from textual.css.scalar import Scalar, Unit
 from textual.css.styles import Styles, RenderStyles
 from textual.dom import DOMNode
+from textual.widget import Widget
 
 
 def test_styles_reset():
@@ -131,3 +135,53 @@ def test_opacity_set_invalid_type_error():
     styles = RenderStyles(DOMNode(), Styles(), Styles())
     with pytest.raises(StyleTypeError):
         styles.opacity = "invalid value"
+
+
+@pytest.mark.parametrize(
+    "size_dimension_input,size_dimension_expected_output",
+    [
+        # fmt: off
+        [None, None],
+        [1, Scalar(1, Unit.CELLS, Unit.WIDTH)],
+        [1.0, Scalar(1.0, Unit.CELLS, Unit.WIDTH)],
+        [1.2, Scalar(1.2, Unit.CELLS, Unit.WIDTH)],
+        [1.2e3, Scalar(1200.0, Unit.CELLS, Unit.WIDTH)],
+        ["20", Scalar(20, Unit.CELLS, Unit.WIDTH)],
+        ["1.4", Scalar(1.4, Unit.CELLS, Unit.WIDTH)],
+        [Scalar(100, Unit.CELLS, Unit.WIDTH), Scalar(100, Unit.CELLS, Unit.WIDTH)],
+        [Scalar(10.3, Unit.CELLS, Unit.WIDTH), Scalar(10.3, Unit.CELLS, Unit.WIDTH)],
+        [Scalar(10.4, Unit.CELLS, Unit.HEIGHT), Scalar(10.4, Unit.CELLS, Unit.HEIGHT)],
+        [Scalar(10.5, Unit.PERCENT, Unit.WIDTH), Scalar(10.5, Unit.WIDTH, Unit.WIDTH)],
+        [Scalar(10.6, Unit.PERCENT, Unit.PERCENT), Scalar(10.6, Unit.WIDTH, Unit.WIDTH)],
+        [Scalar(10.7, Unit.HEIGHT, Unit.PERCENT), Scalar(10.7, Unit.HEIGHT, Unit.PERCENT)],
+        # percentage values are normalised to floats and get the WIDTH "percent_unit":
+        [Scalar(11, Unit.PERCENT, Unit.HEIGHT), Scalar(11.0, Unit.WIDTH, Unit.WIDTH)],
+        # fmt: on
+    ],
+)
+def test_widget_style_size_can_accept_various_data_types_and_normalize_them(
+    size_dimension_input, size_dimension_expected_output
+):
+    widget = Widget()
+
+    widget.styles.width = size_dimension_input
+    assert widget.styles.width == size_dimension_expected_output
+
+
+@pytest.mark.parametrize(
+    "size_dimension_input",
+    [
+        "a",
+        "1.4e3",
+        3.14j,
+        Decimal("3.14"),
+        list(),
+        tuple(),
+        dict(),
+    ],
+)
+def test_widget_style_size_fails_if_data_type_is_not_supported(size_dimension_input):
+    widget = Widget()
+
+    with pytest.raises(StyleValueError):
+        widget.styles.width = size_dimension_input
diff --git a/tests/test_widget.py b/tests/test_widget.py
index 7eee43a54..abc92039c 100644
--- a/tests/test_widget.py
+++ b/tests/test_widget.py
@@ -1,16 +1,22 @@
+from contextlib import nullcontext as does_not_raise
+from decimal import Decimal
+
 import pytest
 
 from textual.css.errors import StyleValueError
+from textual.css.scalar import Scalar, Unit
 from textual.widget import Widget
 
 
 @pytest.mark.parametrize(
-    "set_val, get_val, style_str", [
+    "set_val, get_val, style_str",
+    [
         [True, True, "visible"],
         [False, False, "hidden"],
         ["hidden", False, "hidden"],
         ["visible", True, "visible"],
-    ])
+    ],
+)
 def test_widget_set_visible_true(set_val, get_val, style_str):
     widget = Widget()
     widget.visible = set_val