There is no reason you cannot make the leader clothes any length you want. Some people have short ones they use for crib size quilts, remove those and have another set for a Twin, and one for Standard size and so on.
Personally, my frame is set at 104" and i made my leader clothes the same length. I use them for any/all size quilts.
It is your frame, so make them the size you want. I don't think there are any RULES and no Quilt Police. If you want 120" leaders, go for it!
Happy Quilting.
June in Cincinnati