.45 is my choice and has been for many years.
Thanks.
You might want to try out the average small female on the typical derringer and again on the Glock 30/36. Chances are that she'll find the Glock lighter-recoiling and a lot easier to shoot. For the calibers they come in, derringers have horrible felt recoil and a great deal of muzzle flash, which means that the lady isn't likely to practice with the weapon a great deal. And we all know that practice with one's chosen weapon is *very, very important*.
I figured this out a while back and stopped recommending derringers and revolvers to women, except for those so hopelessly confused by technology that they couldn't be taught how to take apart and clean a Glock. (And yes, there are guys like that, too - seems to be about the same percentage of the population for both genders, so nobody take offense). Other than those, I recommend autopistols.