From my perspective, it depends on the piecing of the blocks in the quilt. I am probably not thinking in "quilt orthodoxy", but I like the overall look to be balanced. So I would audition several strip sizes against the quilt to see what I think looks best and then decide on the width and length of the keys. If the width and/or length of the strips overpower the blocks, I'd make them smaller. If the keys get lost next to the blocks, I'd try wider and/or longer strips. Just my opinion.