struct window_t{
    ...
    void get_size( int& height, int& widht ) const;
};

You can not expose get_size function as is - int is immutable type in Python. So, we need to create a wrapper to the function:

boost::python::tuple get_size_wrapper( const window_t& win ){
    int height(0), width( 0 );
    win.get_size( height, widht );
    return boost::python::make_tuple( height, width );
}

class_<window_t>( ... )
    .def( "get_size", &get_size_wrapper )
    ...
;

Now, after you know how this problem is solved. I will show how this solution could be integrated with Py++.

wrapper_code = \
"""
   static boost::python::tuple get_size( const window_t& win ){
      int height(0), width( 0 );
      win.get_size( height, width );
      return boost::python::make_tuple( height, width );
  }
"""

registration_code = 'def( "get_size", &%s::get_size )' % window.wrapper_alias

mb = module_builder_t( ... )
window = mb.class_( "window_t" )
window.member_function( "get_size" ).exclude()
window.add_wrapper_code( wrapper_code )
window.registration_code( registration_code )

That's all.

C++/Python interfacing

How to expose function, which has hand-written wrapper?