What are people's thoughts on caliber size for this scenario?
It'll be a Springfield XDs, 3.5" barrel. It'll be in my safe by my bed 60% of the time, in the Jeep 30% of the time, and on my person in the backcountry 10% of the time.
Main threat is human, with a little cougar and black bear mixed in. I do sometimes go back home to MT and hunt there where there are Grizzlies. That's about 3 days every other year.
Options: 9mm, 40S&W, or 45ACP.
Capacity decreases by one with each step up in caliber.
I'm primary shooter. Wife is secondary. I'm big; she's small.
If this is a nightstand/backcountry/vehicle gun, why on earth are you looking at small single stacks? This is where a full size service pistol comes in. Add a light, get compatible holsters, and go about your day. Full size have better track records for reliability, are heavier for better recoil management, provide a better sight picture, hold more rounds, and are generally easier to shoot. It will be a bit large in your wife's hands, but .45 is a pretty gentle caliber.
As far as caliber, I love me some .45. But I very rarely shoot .45, and when I do, its in a 1911. Assuming this is not your only gun, but needs to fill these roles, I would have a hard time passing up a G21, and then finding a 10mm barrel, a couple of mags, and a recoil spring assembly. Keep it in .45 for home and general use, swap over to 10mm (and buy real 10mm ammo, not "10mm" loaded down to basically .40 specs) for woods use.
You end up with a cartridge that is passable as a woods gun, but significantly better than anything offered in the 3 calibers you initially listed. Should probably be under $200 to add the caliber to a G21. Can also drop down to 9mm as well, about the same cost.
If you are set on an XDs in the calibers you listed, get the 9mm. Better to have 2 extra rounds than the marginally bigger hole of the .45 with ammo that is hasnt really been given proper attention to recent developments, and .40 is dying, even in LEO circles. There is almost zero practical advantages that anyone can argue against 9mm (with a good round like Fed HST) anymore.