diff --git a/packages/next/src/server/config.ts b/packages/next/src/server/config.ts
index 2856ed1930..951a0f1688 100644
--- a/packages/next/src/server/config.ts
+++ b/packages/next/src/server/config.ts
@@ -771,6 +771,29 @@ function assignDefaultsAndValidate(
     }
   }
 
+  // Validate & normalize experimental.maxPostponedStateSize
+  if (typeof result.experimental?.maxPostponedStateSize !== 'undefined') {
+    const bytes =
+      require('next/dist/compiled/bytes') as typeof import('next/dist/compiled/bytes')
+    const maxPostponedStateSize = result.experimental.maxPostponedStateSize
+    let value: number | null
+
+    if (typeof maxPostponedStateSize === 'number') {
+      value = maxPostponedStateSize
+    } else {
+      value = bytes.parse(maxPostponedStateSize)
+    }
+
+    if (value === null || isNaN(value) || value < 1) {
+      throw new Error(
+        'Max Postponed State Size must be a valid number (bytes) or filesize format string (e.g., "5mb"): https://nextjs.org/docs/app/api-reference/config/next-config-js/max-postponed-state-size'
+      )
+    }
+
+    // Store the normalized value as a number
+    result.experimental.maxPostponedStateSize = value
+  }
+
   // Throw if both Middleware and Proxy config are set.
   if (
     userConfig.experimental?.proxyClientMaxBodySize !== undefined &&