the length of the window is all you need. 64" will leave a thick edge at the bottom, but how often will you unrole them all the way anyway.

If it's just for the top look, just choose something shorter.

Don't get to wide on an outside mount.

actually, if you posted a photo of the window in question, you may get help with placement... Lots of options, as in high on top, above the top trim (mounted on the wall) to look like a valance.

Thanks. Here’s the photo of the 2 windows.