diff --git a/cryptography/hazmat/primitives/interfaces.py b/cryptography/hazmat/primitives/interfaces.py
index 3824bcd..eab48b4 100644
--- a/cryptography/hazmat/primitives/interfaces.py
+++ b/cryptography/hazmat/primitives/interfaces.py
@@ -367,6 +367,12 @@
 
 class DSAPublicKey(six.with_metaclass(abc.ABCMeta)):
     @abc.abstractproperty
+    def key_size(self):
+        """
+        The bit length of the prime modulus.
+        """
+
+    @abc.abstractproperty
     def y(self):
         """
         The public key.