diff --git a/docs/getting_started/user-guide.rst b/docs/getting_started/user-guide.rst
index 48cbdb9..8915738 100644
--- a/docs/getting_started/user-guide.rst
+++ b/docs/getting_started/user-guide.rst
@@ -593,6 +593,18 @@
    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.
+
+   +-----------+------------------------------------+
+   |  KEY_ALG  |        Possible key sizes          |
+   +===========+====================================+
+   |    rsa    |  1024, 2048 (default), 3072, 4096  |
+   +-----------+------------------------------------+
+   |   ecdsa   |            unavailable             |
+   +-----------+------------------------------------+
+
 -  ``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``.