diff --git a/Userland/Libraries/LibWeb/CMakeLists.txt b/Userland/Libraries/LibWeb/CMakeLists.txt
index 328a5b367e..73750ce516 100644
--- a/Userland/Libraries/LibWeb/CMakeLists.txt
+++ b/Userland/Libraries/LibWeb/CMakeLists.txt
@@ -57,6 +57,7 @@ set(SOURCES
     CSS/Selector.cpp
     CSS/SelectorEngine.cpp
     CSS/Serialize.cpp
+    CSS/Size.cpp
     CSS/StyleComputer.cpp
     CSS/StyleProperties.cpp
     CSS/StyleSheet.cpp
diff --git a/Userland/Libraries/LibWeb/CSS/Size.cpp b/Userland/Libraries/LibWeb/CSS/Size.cpp
new file mode 100644
index 0000000000..bdc4ebd5f3
--- /dev/null
+++ b/Userland/Libraries/LibWeb/CSS/Size.cpp
@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibWeb/CSS/Length.h>
+#include <LibWeb/CSS/Size.h>
+#include <LibWeb/CSS/StyleValue.h>
+
+namespace Web::CSS {
+
+Size::Size(Type type, LengthPercentage length_percentage)
+    : m_type(type)
+    , m_length_percentage(move(length_percentage))
+{
+}
+
+CSS::Length Size::resolved(Layout::Node const& node, Length const& reference_value) const
+{
+    return m_length_percentage.resolved(node, reference_value);
+}
+
+Size Size::make_auto()
+{
+    return Size { Type::Auto, Length::make_auto() };
+}
+
+Size Size::make_length(Length length)
+{
+    return Size { Type::Length, move(length) };
+}
+
+Size Size::make_percentage(Percentage percentage)
+{
+    return Size { Type::Percentage, move(percentage) };
+}
+
+Size Size::make_min_content()
+{
+    return Size { Type::MinContent, Length::make_auto() };
+}
+
+Size Size::make_max_content()
+{
+    return Size { Type::MaxContent, Length::make_auto() };
+}
+
+Size Size::make_fit_content(Length available_space)
+{
+    return Size { Type::FitContent, move(available_space) };
+}
+
+Size Size::make_none()
+{
+    return Size { Type::None, Length::make_auto() };
+}
+
+bool Size::contains_percentage() const
+{
+    switch (m_type) {
+    case Type::Auto:
+    case Type::MinContent:
+    case Type::MaxContent:
+    case Type::None:
+        return false;
+    default:
+        return m_length_percentage.contains_percentage();
+    }
+}
+
+}
diff --git a/Userland/Libraries/LibWeb/CSS/Size.h b/Userland/Libraries/LibWeb/CSS/Size.h
new file mode 100644
index 0000000000..bf031d08dc
--- /dev/null
+++ b/Userland/Libraries/LibWeb/CSS/Size.h
@@ -0,0 +1,72 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibWeb/CSS/Length.h>
+#include <LibWeb/CSS/Percentage.h>
+
+namespace Web::CSS {
+
+class Size {
+public:
+    enum class Type {
+        Auto,
+        Length,
+        Percentage,
+        MinContent,
+        MaxContent,
+        FitContent,
+        None, // NOTE: This is only valid for max-width and max-height.
+    };
+
+    static Size make_auto();
+    static Size make_length(Length);
+    static Size make_percentage(Percentage);
+    static Size make_min_content();
+    static Size make_max_content();
+    static Size make_fit_content(Length available_space);
+    static Size make_none();
+
+    bool is_auto() const { return m_type == Type::Auto; }
+    bool is_length() const { return m_type == Type::Length; }
+    bool is_percentage() const { return m_type == Type::Percentage; }
+    bool is_min_content() const { return m_type == Type::MinContent; }
+    bool is_max_content() const { return m_type == Type::MaxContent; }
+    bool is_fit_content() const { return m_type == Type::FitContent; }
+    bool is_none() const { return m_type == Type::None; }
+
+    // FIXME: This is a stopgap API that will go away once all layout code is aware of CSS::Size.
+    CSS::Length resolved(Layout::Node const&, Length const& reference_value) const;
+
+    bool contains_percentage() const;
+
+    CSS::Length const& length() const
+    {
+        VERIFY(is_length());
+        return m_length_percentage.length();
+    }
+
+    CSS::Percentage const& percentage() const
+    {
+        VERIFY(is_percentage());
+        return m_length_percentage.percentage();
+    }
+
+    CSS::Length const& fit_content_available_space() const
+    {
+        VERIFY(is_fit_content());
+        return m_length_percentage.length();
+    }
+
+private:
+    Size(Type type, LengthPercentage);
+
+    Type m_type {};
+    CSS::LengthPercentage m_length_percentage;
+};
+
+}
diff --git a/Userland/Libraries/LibWeb/Forward.h b/Userland/Libraries/LibWeb/Forward.h
index 8a78242603..d9f9510f71 100644
--- a/Userland/Libraries/LibWeb/Forward.h
+++ b/Userland/Libraries/LibWeb/Forward.h
@@ -91,6 +91,7 @@ class ResolutionStyleValue;
 class Screen;
 class Selector;
 class ShadowStyleValue;
+class Size;
 class StringStyleValue;
 class StyleComputer;
 class StyleProperties;