Bug #1734111: document struct.Struct.size.
diff --git a/Doc/library/struct.rst b/Doc/library/struct.rst
index 2f27d13..9cf4eb2 100644
--- a/Doc/library/struct.rst
+++ b/Doc/library/struct.rst
@@ -290,3 +290,8 @@
 
    The format string used to construct this Struct object.
 
+.. attribute:: Struct.size
+
+   The calculated size of the struct (and hence of the string) corresponding
+   to :attr:`format`.
+