I found in selecting a bag I had to consider two things. First, I had to consider the size of my hands. I have very large hands, an XL glove is a very tight fit on my hands. So I need a larger bag just to accommodate my hand size. I tried a smaller bag but it was a real wrestling match for me to get my hand in the bag and find anything. I ended up with a bag that measures about 10 inches wide at the top and is about 8 or 9 inches deep. It is larger than I need just based on what I carry in it but size matters when you have big hands. I know there are a lot of people out there who are glad I didn't grow up to be their proctologist. :haha:
The second consideration is what you plan to carry in your bag. Some folks carry just the bare minimum while others carry everything but the kitchen sink. I fall somewhere in the middle.
So, consider what you will be carrying and how easily you can find what you are looking for in your bag when selecting a size and style.
My bag is large made of plain leather with no embelishments and has two pockets in it. In one, I carry my ball bag, my caps (if I am carrying one of my percussion rifles) or my flint wallet and my patching material. In the other pocket, I carry any tools and cleaning supplies such as some wiping patches and a cleaning jag as well as a jag to remove a lost patch, etc......and maybe a small snack.