diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 24951a6..f89faec 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -458,6 +458,10 @@
  *
  * \note            For RSA, md_alg may be MBEDTLS_MD_NONE if hash_len != 0.
  *                  For ECDSA, md_alg may never be MBEDTLS_MD_NONE.
+ *
+ * \note            In order to ensure enough space for the signature, the
+ *                  \p sig buffer size must be of at least
+ *                  `max(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)` bytes.
  */
 int mbedtls_pk_sign( mbedtls_pk_context *ctx, mbedtls_md_type_t md_alg,
              const unsigned char *hash, size_t hash_len,
@@ -472,6 +476,10 @@
  *                  \c mbedtls_ecp_set_max_ops() to reduce blocking for ECC
  *                  operations. For RSA, same as \c mbedtls_pk_sign().
  *
+ * \note            In order to ensure enough space for the signature, the
+ *                  \p sig buffer size must be of at least
+ *                  `max(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)` bytes.
+ *
  * \param ctx       The PK context to use. It must have been set up
  *                  with a private key.
  * \param md_alg    Hash algorithm used (see notes)
diff --git a/include/mbedtls/rsa.h b/include/mbedtls/rsa.h
index 489f2ed..610e804 100644
--- a/include/mbedtls/rsa.h
+++ b/include/mbedtls/rsa.h
@@ -907,7 +907,8 @@
  *                 the size of the hash corresponding to \p md_alg.
  * \param sig      The buffer to hold the signature. This must be a writable
  *                 buffer of length \c ctx->len Bytes. For example, \c 256 Bytes
- *                 for an 2048-bit RSA modulus.
+ *                 for an 2048-bit RSA modulus. A buffer length of
+ *                 #MBEDTLS_MPI_MAX_SIZE is always safe.
  *
  * \return         \c 0 if the signing operation was successful.
  * \return         An \c MBEDTLS_ERR_RSA_XXX error code on failure.
@@ -954,7 +955,8 @@
  *                 the size of the hash corresponding to \p md_alg.
  * \param sig      The buffer to hold the signature. This must be a writable
  *                 buffer of length \c ctx->len Bytes. For example, \c 256 Bytes
- *                 for an 2048-bit RSA modulus.
+ *                 for an 2048-bit RSA modulus. A buffer length of
+ *                 #MBEDTLS_MPI_MAX_SIZE is always safe.
  *
  * \return         \c 0 if the signing operation was successful.
  * \return         An \c MBEDTLS_ERR_RSA_XXX error code on failure.
@@ -1015,7 +1017,8 @@
  *                 the size of the hash corresponding to \p md_alg.
  * \param sig      The buffer to hold the signature. This must be a writable
  *                 buffer of length \c ctx->len Bytes. For example, \c 256 Bytes
- *                 for an 2048-bit RSA modulus.
+ *                 for an 2048-bit RSA modulus. A buffer length of
+ *                 #MBEDTLS_MPI_MAX_SIZE is always safe.
  *
  * \return         \c 0 if the signing operation was successful.
  * \return         An \c MBEDTLS_ERR_RSA_XXX error code on failure.