Rolf,
Several years back a flask like the one you have illustrated was offered for sale on Ebay as a powder flask. Much discussion ensued and several knowledgeable antique dealers confirmed that these were not powder flasks, but rather were commonly carried by women, suspended from their chatelaines. I disremember what the contents carried in them were.
On the other hand, I have seen a picture of one in a book on the history of guns illustrated as a "powder flask." Whether this is fact or poor research on the author's part, I have no idea.
>1.How big should a powder flask for a pair of cased pistols be?
Most would be in the range of this one....4.5" long and 1" thick.
>2. How thick should the scales be?
I guess it depends on your design and construction.
>3.How are they attached to the metal?
Doing the one you illustrate, think of the rim on a pair of eyeglasses. The metal band retains the scales in a similar manner, however the joint would be soldered rather than screwed. In doing other designs, such as the ovular one I illustrated the metal could be attached to the ivory with screws or pins.
>4.Whats the size of the spout and how is the stopper constructed?
The spout size is proportional to the body size for aesthetic purposes. This is the standard construction for a common flask.
The one you illustrate would pivot on tabs soldered to the spout and would have a captured spring mounted under the pivot.
This is all pretty generic, but you should easily be able to adapt any of the methods to your design, whichever direction you choose to go.