diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index fded1e0..583e8ca 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -350,6 +350,21 @@
    compliant and is retained only for compatibility. The default value of this
    flag is ``rsa`` which is the TBBR compliant PKCS#1 RSA 2.1 scheme.
 
+-  ``KEY_SIZE``: This build flag enables the user to select the key size for
+   the algorithm specified by ``KEY_ALG``. The valid values for ``KEY_SIZE``
+   depend on the chosen algorithm and the cryptographic module.
+
+   +-----------+------------------------------------+
+   |  KEY_ALG  |        Possible key sizes          |
+   +===========+====================================+
+   |    rsa    | 1024 , 2048 (default), 3072, 4096* |
+   +-----------+------------------------------------+
+   |   ecdsa   |            unavailable             |
+   +-----------+------------------------------------+
+
+   * Only 2048 bits size is available with CryptoCell 712 SBROM release 1.
+     Only 3072 bits size is available with CryptoCell 712 SBROM release 2.
+
 -  ``HASH_ALG``: This build flag enables the user to select the secure hash
    algorithm. It accepts 3 values: ``sha256``, ``sha384`` and ``sha512``.
    The default value of this flag is ``sha256``.