how to chose what size pin to use and the difference?

What are you using it for?

Some like to use .029 pins to hunt with and use .019 or .010 pins for target. I like .019 for everything, but you have to make sure they are bright enough for low light conditions if you plan on hunting with them. I like a smaller pin because it allows me to pick out a smaller spot.

Smaller pins for smaller/farther targets. I practice a lot at 50-60 yards so I don't like a pin any bigger than .019" and if it were just for targets I'd go with a .010" though they give up some brightness and durability so I'd never take them hunting. I use .019" for everything.

My 3d sight has four wrapped .10 pins. I need that small a pin for the precision shooting that is needed when trying to hit a dime-sized 11-ring sometimes...

My hunting sight has a wrapped .29 top pin, followed by three wrapped.19 pins. The larger size won't obstruct my target at 20 yards, while the .19's are plenty small enough to do the same at longer distances. My first attempt at this concept was with a .19 followed by three .10s for hunting, but my eyes just aren't good enough to use the .10s in a hunting situation.

Smaller Pins help you aim small so you can miss small.

do the HHA Optimizers come with .019 pins?? I think that is a good pin for hunting.