Look at the knitting sheaths in all the collections and most of them are 2o to 25 cm in length. Then, at the end of the Beamish collection is a big one -- with curved needles -- see the fine Beamish image at http://www.beamishcollections.com/itemsimages/thumnails/161155.jpg. It is 42 cm long. A big one!
I looked at it, and I did not pay attention because so many knitting sheaths were smaller and they worked very well with even 12 inch straight needles.
However, knitting with curved needles is a different technology and requires different tools.
I tried making a bigger knitting sheath. See above. The big one. It is the one that works with those little curved needles. Try an evening of swaving with one of those 20 to 25 cm long knitting sheaths, and the next morning you will wake up with sore fingers.
Really, the little ones do work just fine for straight needles.