Maybe I didn't get you right, but sf::RenderWindow::SetView could be what you are looking for.
If you want to have a 320*240 screen diplayed in a 640*480 window, you could just
write something like:
sf::RenderWindow App(sf::VideoMode(640, 480), "ViewExample");
sf::View View(sf::FloatRect(0, 0, 320, 240));
App.SetView(&View);
Hope that helps.