[RPKESQ]

Unless your wife is very small or has some physical limitation affecting her hand or arm strength to a lower than normal level for a woman, she can learn to shoot anything even up to a large magnum rifle or handgun or a 12 gauge shotgun. That being said there are additional considerations to be addressed.
First we will assume you will start her with a small .22 at very close range (6 feet). The main goals for this stage is to build her confidence and to accustom your wife to the noise and proper basic gun saftey. I try to match the .22 action type to the chosen final solution. That is a revolver if she will end up carrying a larger caliber revolver and of course the same with a semiauto.
So before you choose the action type you need to see what she can physically do with various types. First explain this next process very carefully and before you acutally try to do this. Many women have an extremely difficult time either reaching the trigger on a revolver and being able to pull it through or lack the "pinch" strength to pull a semiauto slide back saftely to clear the weapon.
Now, go to a good gunstore where the salespeople will be very patient and where the salepeople agree to not offer any advice. You are her mentor, she needs a single source of information in the beginning (when she is fully trained she will be able to handle divergent advice). Pick out small caliber handguns (.32s,.38s, .380, small 9mms) in both revovler and semiauto styles.
Carefully note if she has any trouble reaching and pulling the trigger on the small revolvers. yes trigger pulls can be lightened a little on a selfdefense weapon, but not much. Try different brands, the trigger reach varies between brands, models and grips.
Next try the semiautos. She will need to know why she has to cycle the slide plus reach and pull the trigger, so this is something you should have explained before arriving at the gunshop. Again, try different brands, models, etc.
If she has extreme difficulty in both action types, pick a smaller caliber gun (even if you have a little skill you can defend yourself 97% of the time with a .22 in civilian life. No, I am not saying a .22 is optimal, but any gun that you can function with is a million times better than not having a gun when you need one).
Take you time and show great patience in this. Do not become fustrated if it requires multiple trips to the gunshop in order to establish what will work the best initially. You might have to explain things multiple times or repeat the various attempt before an answer develops.
Once the action type is decided on you can proceed with live fire with a .22, slowly progressing in all areas. That is: saftey, grip, trigger control, sight picture, longer range, loading and unloading, etc. Only when she can saftely load, unload, fire with a proper grip and hit a 6 inch target at 25-30 feet consistantly is it time to move up to a larger caliber. Some people progress at a faster or slower pace, but the goals must be met completly before progressing on.
Now at last you are ready to move on to the actual carry piece (which might turn out to be the same as she has been using). The basic operational foundation should be now firmly established and she is ready to start learning firearm based self defense (which is an entirely different subject from learning to handle and function a firearm properly).
Note: any gun can be coated to be almost any color, decide this after the final firearm choice is made.
Feel free to PM me if I can be of any help.