Good knockdown power" and "notcause my wife to be afraid of it"... Since you didn't mention anything about the ability to conceal it, I would absolutely stay away from some of the compacts / subcompacts / J Frame revolvers.

I think Colorado Cajun nailed it.

I'd suggest a medium or full size frame. 9mm, .40 (may have too much 'bark' for her), or a .45 (again, full size to reduce recoil). If your mindset is on a revolver, avoid the super compact J Frames and train with 38spl and carry either +P (my preference) or .357 (quite a snap to them). But first practice with the 38spl.

Lots of quality firearms out there. Go to a range with lots of rental firearms and see what she likes. See what feels good in her hand. See what inspires confidence - really!

Good luck.