How big should a knitting sheath be?

 As I began this exploration, I read a lot of discussion about the size of knitting sheaths in collections. Now I know those folk did not know anything about knitting sheaths.

Consider the 2 knitting sheaths below. Both are favorites of mine. The little one is used with  US 000 needles for fine knitting of "sock" yarns. The big sheath on the right is is used with thicker needles for knitting objects suited to cold environments from 5-ply gansey and  10-ply Aran yarns. 

Sure, big knitters need larger knitting sheaths than small knitters, but knitting fine yarns on thin needles is more comfortably done with a small knitting sheath, while thick yarns knit firmly into a weatherproof fabric is better done on a larger knitting sheath.

I started this adventure focused on objects that provide significant warmth. I needed heavy-duty needles and knitting sheaths. I needed a big, heavy knitting bag to carry what I planned to knit in a week.  These days, working on 1.5 mm needles, a week's knitting is smaller and lighter.  And, my knitting sheath is much smaller.  

These days, I am knitting from cakes of  yarn, that are held on the brass "S" hook (clew, or clue).The weight of the hook and yarn helps to stabilize the knitting sheath. This stabilization is more important with the smaller and more delicate knitting sheaths.  On the other hand, I am starting off with cakes that weigh 2 oz. instead of 8 oz. The small cakes are much more convenient hanging from the clew than the larger cakes of gansey yarn.