The correct height will depend on the installation you make. If your window is too small with too much wall left on all sides, the curtain should only cover the window so that it is proportional. You can place the bar 20 centimeters up the window, 10 or 15 centimeters longer for each side and the curtain should be about 10 to 20 centimeters below the window.
If you have a window that is proportional to the wall, install the rod midway between the top of the window and the ceiling. So it is neither too high nor too low. In this case, the rule for the sides remains the same, from 10 to 15 centimeters for each side beyond the size of the window.
The ideal is that the curtain reaches almost to the floor, but without dragging. If you do not want the curtain to reach the floor, it should follow the same length pattern as the top, becoming proportional.
If you have a window that is proportional to the size of the wall, neither too high nor too low, you can create an elongated effect by placing the curtain from the roof wheel up there at the end of the wall. In this case the curtain should also reach the floor.
We hope to have clarified your main doubts about the curtain rod. Now you just have to take measurements from your window and the wall to buy a rod that is proportional to your space. If you still have questions when decorating, talk to a professional in interior design to ensure the right choice.
For more ideas like this, please also read our article titled dress your windows in spectacular style.