You could use a program named Magpie.
That program enlarges screens from even very old VN's to full screen size while even improve the image quality by applying anisotropic filtering and more. You can define the hot key for full screen and back (I configured as alt-F). The beauty is that it only captures the picture, but does not interfere with the game process itself, making crashes very rare. Of course it can't do wonders, so a old TV ratio (4:3) will not be fitted magically into HD (16:9) without stretching or having black borders left and right (your choice).