Sometimes people use an overwide curtain rod so their curtains can stack back to show all of the window. We have no way of knowing how wide you are hanging the curtains. In other words, it is best if you post a picture

Does your LR have a cornice I think they look best if both formal spaces have them as for using them with sheers I would say no.unless the sheers are very full If you are going to hang drapery in front of the sheers then you decide how wide you want the window to appear and that will be the width as for height about 1/6 the length of the drapes usually works , It would help to see the actual room

Here is photo.. I removed a swag cornice and drapes as they were not my style or color...