diff --git a/lib/mcp/resource.rb b/lib/mcp/resource.rb
index 990156b4..e4e55fcd 100644
--- a/lib/mcp/resource.rb
+++ b/lib/mcp/resource.rb
@@ -5,15 +5,16 @@
 
 module MCP
   class Resource
-    attr_reader :uri, :name, :title, :description, :icons, :mime_type, :meta
+    attr_reader :uri, :name, :title, :description, :icons, :mime_type, :size, :meta
 
-    def initialize(uri:, name:, title: nil, description: nil, icons: [], mime_type: nil, meta: nil)
+    def initialize(uri:, name:, title: nil, description: nil, icons: [], mime_type: nil, size: nil, meta: nil)
       @uri = uri
       @name = name
       @title = title
       @description = description
       @icons = icons
       @mime_type = mime_type
+      @size = size
       @meta = meta
     end
 
@@ -25,6 +26,7 @@ def to_h
         description: description,
         icons: icons&.then { |icons| icons.empty? ? nil : icons.map(&:to_h) },
         mimeType: mime_type,
+        size: size,
         _meta: meta,
       }.compact
     end
diff --git a/test/mcp/resource_test.rb b/test/mcp/resource_test.rb
index fb2254d4..a3d6a1b8 100644
--- a/test/mcp/resource_test.rb
+++ b/test/mcp/resource_test.rb
@@ -49,5 +49,23 @@ class ResourceTest < ActiveSupport::TestCase
 
       assert_equal meta, resource.to_h[:_meta]
     end
+
+    test "#to_h omits size when nil" do
+      resource = Resource.new(uri: "file:///test.txt", name: "resource_without_size")
+
+      refute resource.to_h.key?(:size)
+    end
+
+    test "#to_h includes size when present" do
+      resource = Resource.new(uri: "file:///test.txt", name: "resource_with_size", size: 12_345)
+
+      assert_equal 12_345, resource.to_h[:size]
+    end
+
+    test "#to_h includes size when zero" do
+      resource = Resource.new(uri: "file:///empty.txt", name: "empty_resource", size: 0)
+
+      assert_equal 0, resource.to_h[:size]
+    end
   end
 end