Bag sizes & ratio’s
A good size for a bag is one that holds between 4-5 litres of air, this should be able to be filled with about a 4 to 1 ratio, 4 pumps of the bellows should just about fill the bag, always keep the bag full when playing.
A bigger bag will take more pumps filling it but once full will work at the same rate as a smaller one will, IE the reed/s will use the same amount of air whatever size bag is fitted.
Like Uilliam I prefer the smaller bag, 28-1/2" X 10", a bigger deeper bag has a slight dissadvantage in that when playing with others, if you let it go down, they will be waiting for you longer as you fill her up again.
The most important thing is that it fits under your arm comfortably, sometimes the deeper bag tends to push downwards and away from you when its full, this stops you getting your arm around it properly, when playing you should not need to keep re-positioning the bag, you should be able to play and forget its there kind of thing, a persnal fitting is sometimes needed for the shorter or vertically challenged person.
The way it is positioned against your arm is also important, if you are squeezing it off center say near the front part of the bag where the neck starts, then the overall travel of your arm will be limited to 2 to 3" of the travel from the side of your body.
If your arm it centered nearer the middle of the main sausage the travel is increased by a couple more inches giving better control, the nearer your arm lies to your body the less leverage you will have.
Also, bellows that are not efficient are a bain, if the bag takes much more than a 4 to 1 ratio to fill it, you can soon get tired fingers, because the faster you pump the more your hands and fingers are constantly changing position on the chanter causing stresses on unclimatised tendons especially for beginners who’s body is not used to being moved in a new/strange way, a poor set of bellows is like a leaking bag.
Davy.