Jack,
I have not made a lot of horns, but in my case I tend to make them like they were made in the period of the gun for which they will be used. For most of the 18th century guns, that meant the horns are bigger than what one would need for hunting, though they hold plenty of powder for target shooting.
For deer hunting, I used one of the pistol flasks shown in the link below as it carried 2 ounces or 875 grains of powder and that was MORE than enough for a day's deer hunt. Since I usually carried my horn separate from the pouch strap, I would just slip it into my shot pouch/hunting bag and not carry my powder horn. When it got cold enough to need to use gloves, this was a very handy way to carry my powder.
https://www.trackofthewolf.com/Categories/PartDetail.aspx/69/1/FLASK-BAG
Years later, our recently deceased forum member Brown Bear used to talk about doing the same thing with a small cow horn powder horn he also carried in his hunting pouch.
Gus