I just won this yarn holder on ebay (this photo is from the listing) for my son who loves cats and is just learning to knit. I bought another in a different pattern for my partner's birthday (he crochets) and I'm using it until I can find some for me. I like these so much better than the hard plastic and cardboard types. This sort I can squish up and stuff into my knitting bag when I'm out and about with a project. The holder is about 6" tall and 4" in diameter with a zippered top and a hole for the wool.
For those who've not used a wool holder, they are nice because they keep the ball under control. When the ball is almost used up and is lightweight, the holder keeps it weighted down so the wool isn't bouncing around. I think they're very practical.