I started out roughly 8 years ago with quite large homemade leather pouches on my first board-cut slingshots, but gradually made them more in accordance with the ammo size being used. The leather I use came in bags of cut leather remnants sold by the fabrics department of a large retail store: plenty of stock for years to come.
The 2018 slingshot competition in Italy was when I first tried the Chinese slingshots with their microfiber mini-pouches calibrated for 7 to 8 mm steel ammo, which is apparently the mostly used caliber during competitions in China - doubtlessly thanks to its flat trajectory, but also because required band draw weights can be kept to low levels i.e less shooting fatigue during training sessions. In contrast to leather pouches, most microfiber pouches do not appear to lose their initial shape and length under repeated tension. Leather has a certain degree of natural elasticity, and deforms quite rapidly under tension, depending on its type and thickness.
From a purely performance related point of view, assuming identical shapes and weight, it seems to me that there should be almost no difference between microfiber and leather pouches, albeit that the significantly reduced elasticity of microfiber should technically increase the power output ever so slightly - something that would need to be tested under specific conditions by the number enthusiasts among you.
From my personal experience, pouch length and size should match the caliber of the steel ammo (or other suitable slingshot ammo) as closely as possible. An oversized pouch increases the potential for aerodynamic flutter, and thus a possibility of smaller sized (lighter) ammo moving off-center inside the pouch after the release, with associated risks of erratic ammo flight and potential fork hits. A larger pouch with a strong band set used for excessively light ammo is also a recipe for rather painful hand slaps. The "one size fits all" does not apply to slingshot pouches, like for tubes and flat bands in the essential draw weight (power) to caliber (weight) equation for optimized performance. Moreover, a narrow fork with a super wide pouch generally spells trouble too.
I use homemade leather pouches to shoot larger ammo of different kinds, but really do like the small Chinese microfiber pouches when it comes to 10 yard target shooting, as they enable a precise and consistent thumb and index finger pinch without deforming under elastic tension, and are perfectly calibrated for the 7 to 8 mm ammo I use. They are cheap to purchase too.
I should add that the center holes of some of the Chinese microfiber pouches are too small to efficiently center the steel ammo they are intended for, which will result in erratic ammo flight; using a leather hole puncher to increase the hole diameter very slightly will work wonders in accuracy if you are faced with that problem. A good test to determine a good pouch center hole size is to place the steel ammo concerned on the center hole with the pouch held upright, and then changing the angle of the pouch sideways to see at what point the ammo rolls off.
A pouch center hole roughly one third of the diameter of round ammo ammo being used yields good results from my experience.
My 2 cents worth... B)