Just an opinion, but bigger is better IMO. I for one really don't want to shoot anybody and if the sight of a firearm makes them reverse direction and take off running, great. Low light or dark, a sub compact isn't going to intimidate anybody.
IMO lady sized or sub compact firearms aren't the best choice, if you have a choice.
Best choice IMO would be a stainless or nickle, full sized revolver, maybe a .357 loaded with 38 spl 110 grain hollow points. In low light, a four inch barrel is going to have an impressive ball of flame come out the end of the barrel. Why not use every edge you can.
Or a slug gun loaded with most anything, at 20 feet it really doesn't make a whole lot of difference with a shotgun.
I'm not saying don't shoot, if you raise it to sight level your next movement is to pull the trigger.
I was a firearms instructor for lady's home defense at our local club. Some tricks you might try or observations you may use. Most of the ladys are going to flinch. Recoil seems to upset them more than the flash or a boom does.
And for what it is worth, teaching them to aim for the crotch seems to produce better results than center of mass. You can think this anyway you want, but it seems to more fact than fantasy. I taught mostly shotgun and had much better results after noticing the crotch shots would change the whole mood of the training season and got much better results overall. I think it is a subliminal wish in the back of most ladys minds anyway, use it.