For me they just sort of end up whatever size they end up.
I take the really narrow ones (like in the picture) and braid them, then braid the braids, then braid THOSE braids, then tie knots into it and give it to my dogs to fight over. It's so cute, they know when they see me braiding and knotting I'm making them a toy and they get all excited about it (even though they have a half-dozen at a time, at least, the NEW one is the exciting one). I also use them as string to play with the cats, and I have a jar of them set aside that I use as "ribbon" when I'm wrapping hand-made gifts. I like the garden ties idea too but we can't seem to get a garden going at this house!
The wider ones that have a little fabric left on them, in the past have often ended up in crazy quilts, but I've started saving them to the side lately; I have in mind trying those out for those fabric-wrapped rope bowls I've seen people making.