Add documentation for List.size()

src/sphere/List.h

@@ -102,6 +102,13 @@ template<class T> class List {
     return current->value;
   }
 
+  /*! \fn length()
+   * \brief Get the number of elements in the list.
+   *
+   * Get the number of elements currently stored in the list.
+   *
+   * \return int: the size of the list.
+   */
   int length() {
     return size;
   }