Most rooms I walk into have the same small problem: the curtain rod sits right on top of the window frame, so the window looks short and squat and the whole wall feels lower than it is.
Try this instead. Hang the rod 10 to 15 cm above the frame, or close to the ceiling if the gap is small. Extend it 15 to 25 cm past the frame on each side, so when the curtains are open they sit on the wall, not over the glass. Then let the panels fall to the floor — just kissing it, not puddling, unless it is a formal room.
Three things change at once. The window reads taller, because your eye follows the fabric up. More daylight comes in, because the open panels are not eating the edges of the glass. And the wall feels finished, because the rod line ties into the ceiling instead of chopping the wall in half.
Two mistakes to avoid. Do not stop the panels at the sill — that shortens everything below them. And do not buy panels exactly as wide as the window; you want enough fullness that they still gather when closed, roughly one and a half to two times the rod width in fabric.
It is a one-afternoon fix, no tools beyond a drill and a level, and it is usually the difference between a room that looks rented and one that looks considered.