diff --git a/include/psa/crypto_extra.h b/include/psa/crypto_extra.h
index f48c087..9bc68f6 100644
--- a/include/psa/crypto_extra.h
+++ b/include/psa/crypto_extra.h
@@ -42,6 +42,13 @@
      PSA_EXPORT_KEY_PAIR_OR_PUBLIC_MAX_SIZE : PSA_CIPHER_MAX_KEY_LENGTH)
 #endif /* !MBEDTLS_PSA_STATIC_KEY_SLOT_BUFFER_SIZE*/
 
+/* Define the size of the each key slot buffer. */
+#if defined(MBEDTLS_PSA_STATIC_KEY_SLOTS)
+#define MBEDTLS_PSA_KEY_BUFFER_MAX_SIZE     MBEDTLS_PSA_STATIC_KEY_SLOT_BUFFER_SIZE
+#else
+#define MBEDTLS_PSA_KEY_BUFFER_MAX_SIZE     SIZE_MAX
+#endif
+
 /** \addtogroup attributes
  * @{
  */