diff --git a/docs/specification/server/resources.md b/docs/specification/server/resources.md
index b2212f3..9eddcb3 100644
--- a/docs/specification/server/resources.md
+++ b/docs/specification/server/resources.md
@@ -246,6 +246,7 @@ A resource definition includes:
 - `name`: Human-readable name
 - `description`: Optional description
 - `mimeType`: Optional MIME type
+- `size`: Optional size in bytes
 
 ### Resource Contents
 
diff --git a/schema/schema.json b/schema/schema.json
index e8b8c78..d360f7b 100644
--- a/schema/schema.json
+++ b/schema/schema.json
@@ -1505,6 +1505,10 @@
                     "description": "A human-readable name for this resource.\n\nThis can be used by clients to populate UI elements.",
                     "type": "string"
                 },
+                "size": {
+                    "description": "The size of the resource in bytes before encoding or tokenization, if known.\n\nThis can be used by Hosts to display file sizes and estimate context window usage.",
+                    "type": "integer"
+                },
                 "uri": {
                     "description": "The URI of this resource.",
                     "format": "uri",
diff --git a/schema/schema.ts b/schema/schema.ts
index eddca23..589fb53 100644
--- a/schema/schema.ts
+++ b/schema/schema.ts
@@ -443,6 +443,13 @@ export interface Resource extends Annotated {
    * The MIME type of this resource, if known.
    */
   mimeType?: string;
+
+  /**
+   * The size of the raw resource content, in bytes (i.e., before base64 encoding or any tokenization), if known.
+   *
+   * This can be used by Hosts to display file sizes and estimate context window usage.
+   */
+  size?: number;
 }
 
 /**