Have you seen the plastic heat deflectors? We used them under our bedroom windows to prevent that same problem. They're really cheap, $5 or $6 at Lowe's or HD. Then you could get your 84" panels.
Dee

Here is a link that might be useful: Heat deflectors

The length of the window doesn't matter (in most cases). What is the distance from the top of the window frame to the floor? Depending on that height and where you place the rod, you may want 84" long drapes or you may want 96" drapes that have been shortened a few inches.

One way to get around the register problem is to use stationary panels to frame the window and then some form of blind for privacy and light control.

Heat deflectors will not work as our registers are slanted. I'm still wondering WHAT SIZE WINDOWS do you use 63" long panels on? Anyone have 63" long panels???

Sorry to offend you. Will not try again.

kaiser, can you post pictures of your windows and registers? It will be easier to help if we can see them.
Like graywings I would also frame the window with stationary floor length panels that hopefully will not be in the way of the registers and use blinds on the windows for privacy and light control.

I have 63" long panels on several windows upstairs. I'm not sure of the window size, but the 63" goes from the rod to the apron trim - just a few inches below the sill.

If you get a 63" panel and it's too long, why not just hem it to 48"? Or Country Curtains catalog does sell several styles in shorter lengths like 52", 48" and 44", though nothing that short with pinch pleats.

Pictures would definitely help.

I'm still wondering WHAT SIZE WINDOWS do you use 63" long panels on?

You don't see many 63" long panels any more. They were widely available many years ago (1960s maybe?) when the fashion was to have the drapes stop at or just below the sill.

There are no hard and fast rules about what you can do with window treatments. You can use a 63 inch long drapery on any size window that you want . . . as long as you are happy with the way it looks.

Back in the day when I used tie-back ruffled priscillas, I usually bought 63" length for bedrooms even tho my windows were about 42 inches long because (1) they were cheaper, and (2)I did not care that they were not floor length because my bedrooms are so small, there was usually a piece of furniture or part of the bed under the window. I actually preferred that length for tie-backs, which made them fall about halfway between the bottom of window and floor. I have never liked curtains the same length as my window, except over a sink. In my living/dining areas I usually went with floor length in this style (does anyone remember the Dorothy's Ruffled Originals). That was me.

Now I prefer panels, shirred to the side, covering very little of the window, hanging to the floor (not puddling).

Like Greywings said, it is just whatever you prefer.

Tuesday

63" is available as a standard size because it works a w/a fairly standard window size (particularly in an older house) where the curtain is mounted on or around the upper trim and the hem falls to or around the sill or apron below the sill. If the top of the window is 84" from the floor, a 63" curtain will hang 21" from the floor. On many windows in modern houses with a 30-33" sill height this will put the curtain length at some point on the wall below the window. This is a look that currently bothers people, but it used to be seen in decorating books and magazines through the 50s or so.

I am certainly not a professional; however, I believe you can make any length classy! I am currently making lined/pleated draperies for a large picture window where my piano is placed and I am thinking about changing it up and making them 63" in length! All fashion, whether it be in clothing or home decoration, comes and goes. Do whatever makes you happy!!!

The shorter lengths can also be nice in kids rooms where they are either playing on the floor or you don't want them to have the extra fabric to put dirty hands on or pull.