diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 2fdc4c1fc..d0d7ac0f8 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -104,37 +104,54 @@ typedef struct mbedtls_pk_rsassa_pss_options
 /**
  * \brief           Maximum size of a signature made by mbedtls_pk_sign().
  */
-/* This fallback value is used if there is no software signature support.
- * This is possible even if check_config.h is included, for example if
- * MBEDTLS_ECDH_C is enabled but neither MBEDTLS_ECDSA_C  nor MBEDTLS_RSA_C.
- * Use MBEDTLS_MPI_MAX_SIZE which is the maximum size than an RSA-alt
- * implementation can produce, assuming that MBEDTLS_MPI_MAX_SIZE is set
- * correctly. This is not necessarily the best choice of size and it may
- * change in future versions. */
-#define MBEDTLS_PK_SIGNATURE_MAX_SIZE MBEDTLS_MPI_MAX_SIZE
-#if defined(MBEDTLS_RSA_C) &&                                   \
+/* We need to set MBEDTLS_PK_SIGNATURE_MAX_SIZE to the maximum signature
+ * size among the supported signature types. Do it by starting at 0,
+ * then incrementally increasing to be large enough for each supported
+ * signature mechanism.
+ *
+ * The resulting value can be 0, for example if MBEDTLS_ECDH_C is enabled
+ * (which allows the pk module to be included) but neither MBEDTLS_ECDSA_C
+ * nor MBEDTLS_RSA_C nor any opaque signature mechanism (PSA or RSA_ALT).
+ */
+#define MBEDTLS_PK_SIGNATURE_MAX_SIZE 0
+
+#if ( defined(MBEDTLS_RSA_C) || defined(MBEDTLS_PK_RSA_ALT_SUPPORT) ) && \
     MBEDTLS_MPI_MAX_SIZE > MBEDTLS_PK_SIGNATURE_MAX_SIZE
+/* For RSA, the signature can be as large as the bignum module allows.
+ * For RSA_ALT, the signature size is not necessarily tied to what the
+ * bignum module can do, but in the absence of any specific setting,
+ * we use that (rsa_alt_sign_wrap in pk_wrap will check). */
 #undef MBEDTLS_PK_SIGNATURE_MAX_SIZE
 #define MBEDTLS_PK_SIGNATURE_MAX_SIZE MBEDTLS_MPI_MAX_SIZE
 #endif
+
 #if defined(MBEDTLS_ECDSA_C) &&                                 \
     MBEDTLS_ECDSA_MAX_LEN > MBEDTLS_PK_SIGNATURE_MAX_SIZE
+/* For ECDSA, the ecdsa module exports a constant for the maximum
+ * signature size. */
 #undef MBEDTLS_PK_SIGNATURE_MAX_SIZE
 #define MBEDTLS_PK_SIGNATURE_MAX_SIZE MBEDTLS_ECDSA_MAX_LEN
 #endif
-#if defined(MBEDTLS_USE_PSA_CRYPTO) && \
-    PSA_ASYMMETRIC_SIGNATURE_MAX_SIZE + 11 > MBEDTLS_PK_SIGNATURE_MAX_SIZE
+
+#if defined(MBEDTLS_USE_PSA_CRYPTO)
+#if PSA_ASYMMETRIC_SIGNATURE_MAX_SIZE > MBEDTLS_PK_SIGNATURE_MAX_SIZE
 /* PSA_ASYMMETRIC_SIGNATURE_MAX_SIZE is the maximum size of a signature made
- * through the PSA API in the PSA representation.
- * The Mbed TLS representation is different for ECDSA signatures:
+ * through the PSA API in the PSA representation. */
+#undef MBEDTLS_PK_SIGNATURE_MAX_SIZE
+#define MBEDTLS_PK_SIGNATURE_MAX_SIZE PSA_ASYMMETRIC_SIGNATURE_MAX_SIZE
+#endif
+
+#if PSA_VENDOR_ECDSA_SIGNATURE_MAX_SIZE + 11 > MBEDTLS_PK_SIGNATURE_MAX_SIZE
+/* The Mbed TLS representation is different for ECDSA signatures:
  * PSA uses the raw concatenation of r and s,
  * whereas Mbed TLS uses the ASN.1 representation (SEQUENCE of two INTEGERs).
  * Add the overhead of ASN.1: up to (1+2) + 2 * (1+2+1) for the
  * types, lengths (represented by up to 2 bytes), and potential leading
  * zeros of the INTEGERs and the SEQUENCE. */
 #undef MBEDTLS_PK_SIGNATURE_MAX_SIZE
-#define MBEDTLS_PK_SIGNATURE_MAX_SIZE ( PSA_ASYMMETRIC_SIGNATURE_MAX_SIZE + 11 )
+#define MBEDTLS_PK_SIGNATURE_MAX_SIZE ( PSA_VENDOR_ECDSA_SIGNATURE_MAX_SIZE + 11 )
 #endif
+#endif /* defined(MBEDTLS_USE_PSA_CRYPTO) */
 
 /**
  * \brief           Types for interfacing with the debug module