If you're struggling trying to figure out where to install a curtain, don't worry, I got you!
Avoid positioning your curtain rod right above the window frame, instead, install the rod around 10cm (4 inch) below the ceiling. This will make the room seem taller.
Mount the rod around 15cm (6 inch) wider on each side of the window, this way when the curtains are pulled back they won’t block any light.
I think the length of a curtain is up to one’s personal taste. I prefer it to be around 1cm (1/2 inch) on top of the floor for functionality, as it allows for mopping, and for aesthetics, as it creates the illusion that the curtain is touching the floor.
(You might want to avoid having curtains that touch the floor if you have a robot cleaner).
Hope this was of help ✌🏼