Since you have pretty much settled on either the Myvi or Picanto without any clear preference for either, the safest way to play this would be to actually take your mom for a test drive in both cars and let her choose. In that way, the decision of choice is her own and she will have to live with what she chose.
There are a few reasons why it is sensible to do this.
1. There is money involved.
2. Old people are fickle. They often don't really know what they want in their grumpy moods. The older people get, the more they tend to be childish. Not saying your mom is such, but take precautions all the same. Does your mom have a real need of a new car or is she just being difficult ? I know plenty of old people still driving large sedans with no complaints.
3. If you look at it honestly, a Wira is not appreciably bigger than either a Myvi or a Picanto. The size difference between these models is hardly any justification to dump a perfectly good car and spend the money to buy a new car. If she honestly really cannot handle a Wira, maybe she's already at the point where even a Myvi or Picanto cannot be of much help. Just something you need to consider objectively.
In any case, if she still insist on the new smaller car, don't buy one for her without first letting her test drive the options you can afford. If you let her make the choice, she will have to live with the decision because she was responsible for picking that particular model amongst all the choices she test drove. She cannot come back later and say something like "Aiya. Ah Chai ! Why you pick that car for me ? It don't suit me." Like I said, old people can be fickle. Make sure you be the smart son and protect yourself.