diff --git a/branca/utilities.py b/branca/utilities.py
index 2d25e11..0db25f3 100644
--- a/branca/utilities.py
+++ b/branca/utilities.py
@@ -10,6 +10,7 @@
 import json
 import math
 import os
+import re
 import struct
 import zlib
 
@@ -387,19 +388,22 @@ def _camelify(out):
 
 
 def _parse_size(value):
-    try:
-        if isinstance(value, int) or isinstance(value, float):
-            value_type = "px"
-            value = float(value)
-            assert value > 0
+    if isinstance(value, (int, float)):
+        return float(value), "px"
+    elif isinstance(value, str):
+        # match digits or a point, possibly followed by a space,
+        # followed by a unit: either 1 to 5 letters or a percent sign
+        match = re.fullmatch(r"([\d.]+)\s?(\w{1,5}|%)", value.strip())
+        if match:
+            return float(match.group(1)), match.group(2)
         else:
-            value_type = "%"
-            value = float(value.strip("%"))
-            assert 0 <= value <= 100
-    except Exception:
-        msg = "Cannot parse value {!r} as {!r}".format
-        raise ValueError(msg(value, value_type))
-    return value, value_type
+            raise ValueError(
+                f"Cannot parse {value!r}, it should be a number followed by a unit.",
+            )
+    else:
+        raise TypeError(
+            f"Cannot parse {value!r}, it should be a number or a string containing a number and a unit.",
+        )
 
 
 def _locations_mirror(x):
diff --git a/tests/test_utilities.py b/tests/test_utilities.py
index f2aeb9b..e51d3a2 100644
--- a/tests/test_utilities.py
+++ b/tests/test_utilities.py
@@ -104,3 +104,31 @@ def test_color_avoid_unexpected_error():
         for n in [str(color_brewer_minimum_n), float(color_brewer_minimum_n), "abc"]:
             with pytest.raises(TypeError):
                 ut.color_brewer(sname, n)
+
+
+@pytest.mark.parametrize(
+    "value,result",
+    [
+        (1, (1.0, "px")),
+        ("1 px", (1.0, "px")),
+        ("80 % ", (80.0, "%")),
+        ("100%   ", (100.0, "%")),
+        ("3 vw", (3.0, "vw")),
+        ("3.14 rem", (3.14, "rem")),
+    ],
+)
+def test_parse_size(value, result):
+    assert ut._parse_size(value) == result
+
+
+@pytest.mark.parametrize(
+    "value",
+    [
+        "what?",
+        "1.21 jigawatts",
+        ut._parse_size,
+    ],
+)
+def test_parse_size_exceptions(value):
+    with pytest.raises((ValueError, TypeError)):
+        ut._parse_size(value)