Sounds pretty!
I think I'd make them at least twice the width of the chair back and twice as long as what hangs down the front -- i.e. same amount hanging over the back as is hanging over the front of the chair. That way, if there is ever any fading, you can just switch which part of the quilt is over the back and which part can be seen from the front.
Making them at least twice as wide will at least make them usable at some point in their lives, even if they have faded too much for the decorative use.
Jan in VA