I'm only making a guess here, as I have not tried it yet.
But SFML has a feature called RenderTexture, which you can draw onto instead of the window. By doing this, you should be able to draw your scene onto the render texture, use it as a texture and render onto a larger rectangle and then render onto a smaller rectangle.
Again, I'm not sure, but these forums are not heavy on activity. If I'm completely off track here, try the IRC or Discord channel