fix: add missing size field to ResourceSchema

The MCP specification defines an optional `size?: number` field on the
Resource interface (added in spec revision 2025-06-18), but the Zod
schema was missing it. This adds the field so servers can declare
resource sizes and hosts can display file sizes / estimate context
window usage.

Fixes modelcontextprotocol#1573

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: olaservo

.changeset/add-resource-size-field.md

@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/core': patch
+---
+
+Add missing `size` field to `ResourceSchema` to match the MCP specification

packages/core/src/types/types.ts

@@ -915,6 +915,13 @@ export const ResourceSchema = z.object({
      */
     mimeType: z.optional(z.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: z.optional(z.number()),
+
     /**
      * Optional annotations for the client.
      */