If the recoil of the 45 is too much for her...the 40 probably will be too.
I'm all for the bigger calibers...if they made a concealable 50 AE, I'd carry it

, but as the old saying goes...a hit with a 22 is better than a miss with a 44.
I worked my wife up to the 45acp...took about 4 years. First was 380, then 9mm, borrowed her brothers 40, then she bought the XD 45c herself. I loaded her a bunch of ammo (1,000 rounds...not really soft, but not like my loads either) and just let her have at it with those until they were gone...when they were gone, I didn't tell her...I just loaded her up with my loads (not +P, but near max)...she never even noticed the difference. (kind of a mind game)
Then one day, she finally told me she was ready to start shooting my loads...I told her she had been shooting them for several months ;D
Now...she is giving me hell, because hers (XD 45c) is bigger than mine (G36) :-[
Everybody is an individual though...if your wife doesn't want recoil, it may be best to start at 9mm and work up (I don't trust the 380 as a defense weapon)