For a smaller person this may be less intimidating to practice with. Which means they would be more comfortable using it should the situation arise. 20g or even 410 will get the job done. I prefer 2 3/4 12ga 00buck. I have some 3.5 00 buck but you reduce the amount of capacity with 3.5s.
I was at a crime scene where a woman shot her boy friend with a 410 at about 10ft the wad penetrated the guys face. You can imagine what the 00buck did.